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U. S. DEPARTMENT OF AGRICULTURE, 

BUREAU OF SOILS— BULLETIN No. 32. 
MILTON \A/-HITNEY, Chief. 



THE ABSORPTION OF PHOSPHATES AND 
POTASSIUM BY SOILS. 






OSWALD SCHREINER and GEORGE H. FAILYER. 




WASHINGTON: 

GOVERNMENT PRINTING OFFICE, 




lAonfl' 






LETTER OF TRANSMITTAL. 



U. S. Department of Agriculture, 

Bureau of Soils, 
Was/imgfon, D. C, February 23, 1906. 
Sir: I have the the honor to transmit herewith the manuscript of a 
technical paper entitled "The Absorption of Phosphates and Potas- 
sium by Soils," and to recommend that it be published as Bulletin No. 
32 of the Bureau of Soils. 

Respectfully, Milton Whitney, 

CJtlef of Bureau. 
Hon. James Wilson, 

Secretary of Agriculture. 
2 



^^■^ ^1/SOfi 



PREFACE. 



It has long- been known that soils possess the power of selectively 
absorbing to varying extents the several constituents from a mixture 
in solution, and that even strong salts may in this way be decomposed. 
For instance, potassium is generally absorbed more than chlorine when 
a soil is treated with a solution of potassium chloride. This has been 
explained in the past as resulting from a simple metathetical reaction 
between the potassium salt and other salts already present in the soil. 
This explanation is now known to be unsatisfactory, for, while other 
bases can be found in the resulting solution, they are not present in 
quantities equivalent to the potassium removed and the solution is 
generally acid. Moreover, precisely similar results can be obtained 
with other absorbent media which do not contain any bases to replace 
the potassium. The further fact has been established that an absorb- 
ing medium has a limited capacity for taking up any particular sub- 
stance or constituent, and this capacity, or saturation limit, is generally 
independent of any simple molecular ratio between the absorbed sub- 
stance and any component or constituent of the absorbing medium, 
organic substances, such as dyes, being absorbed in precisely the same 
way as mineral solutes. 

The absorption phenomena resemble in many respects well-known 
solution phenomena, and there is a distribution of the substance which 
is being absorbed between the solid absorbing medium and the liquid 
medium, the resulting concentration of the latter depending upon the 
amount of the absorbed material in the solid. Whether or not there 
is a definite law governing the distribution is a matter of controversy, 
experimental evidence for and against being recorded in the literature. 
The reverse process of the leaching of a soil or solid containing an 
absorbed substance shows similar distribution phenomena between the 
solid and liquid media. 

From these observations it follows that absorption is not merely the 
result of simple metathesis, although such a reaction may be involved 
in any given case. So may be involved adsorption or surface con- 
densation, the formation of solid solutions, or the formation of new 
molecular species, it being often difficult if not impossible definitely 
to determine the cause or causes in any given case. But however 

3 



4 PREFACE. 

complex the ultimate causes of absorption, the law ^overnin^ the 
effect itself is one of the utmost simplicity, as the following pages will 
show, and the reverse process of the removal of absorbed constituents 
by leaching appears to follow a law similar to the law for absorption. 
The differential equation describing the law is identical in form with 
the equation describing rate of solution. 

An important observation indicated by this study is that when a 
soil containing phosphates or potassium is leached, after a certain 
small amount of leaching has taken place, the subsequent leachings are of 
practically constant small concentration, no matter how readily soluble 
the phosphatic or potassium compounds in the soil may be in them- 
selves. This suggests a very close analogy to the case of a slightly 
soluble solid in contact with a saturated solution of itself. It is much 
more probable, however, that there is actually a diminution of the 
concentration in successive leachings, although a diminution which 
would not be appreciated over any range that would ever be realized 
either in the laboratory or in nature. 

That the solubility of the absorbed substance is not a primary factor 
in the phenomena is shown by the action of soils toward certain dyes 
which can be readily leached out with alcohol but not with water, 
although as soluble or even more soluble in the latter as they are in 
the former. A very interesting case has been cited by Walker and 
Appleyard and by van Bemmelen, where picric acid is absorbed by 
silk more readily from an aqueous solution than from an alcoholic 
solution and not at all from a benzene solution, although picric acid is 
more soluble in alcohol than in either water or benzene. 

It is thus made manifest that the absorptive power of the soil is to 
a much larger extent than any other the controlling factor in regu- 
lating the concentration of the soil solution and that, in the absence of 
some exceptional disturbing cause, the concentration will remain prac- 
tically constant. It further appears that different soils will yield 
solutions of the same order of concentration. This absorption study 
confirms in a positive way conclusions regarding the concentration of 
the soil moisture, which have alread}'^ been advanced in former bulle- 
tins of this Bureau, but* which were reached by entirely different lines 
of reasoning and evidence. These conclusions, of obvious funda- 
mental importance, furnish a new point of view for the study of the 
relations of plant growth to the soil and the development of a rational 
system of fertilizer usage which may reasonably be expected to yield 
results of great practical importance in soil management. 

Frank K. Cameron. 



CONTENTS, 



Page. 

Introduction 7 

Description of the soils 8 

Description of the apparatus 11 

Eemoval of phosphate by water 13 

Absorption of phosphate from a solution of monocalcium phosphate 15 

Removal of absorbed phosphate by water 18 

Graphical representation and discussion of the results obtained with mono- 
calcium phosphate 21 

Absorption of phosphate from a solution of disodium phosphate and removal 

of the absorbed phosphate by water 24 

Graphical representation and discussion of the results obtained with sodium 

phosphate 28 

Absorption of potassium from a solution of potassium chloride 31 

Eemoval of absorbed potassium by water 34 

Graphical representation and discussion of the results obtained with potas- 
sium chloride 35 

Summary 38 

5 



ILLUSTRATIONS. 



Page. 
Fig. 1. Apparatus used in the absorption experiments 12 

2. Solution curves. Absorption of phosphate by soils from a solution of 

monocalcium phosphate and the removal of the absorbed phosphate 

by water 22 

3. Soil curves. Absorption of phosphate by soils from a solution of 

monocalcium phosphate and the removal of the absorbed phosphate 

by water - 22 

4. Soil curves. Absorption of phosphate by soils from a solution of 

disodium phosphate and the removal of the absorbed phosphate by 
water 29 

5. Solution curves. Absorption of potassium by soils from a solution of 

potassium chloride and the removal of the absorbed potassium by 
water 36 

6. Soil curves. Absorption of potassium by soils from a solution of 

potassium chloride and the removal of the absorbed potassium by 
water 36 

6 



THE ABSORPTION OF PHOSPHATES AND 
POTASSIUM BY SOILS. 



INTRODUCTION. 



In soils there is present the mineral debris of rock degradation and 
decomposition, ^his mineral matter forms b}'^ far the greater part 
of the soil, the quantity of organic material being comparatively 
small, although agriculturally of great significance. In the formation 
of soils b}^ rock degradation numerous small fragments of the original 
rock persist to a greater or less extent in the soil. This has been 
frequently shown, notably by the recent work of Delage and Lagatu, 
and has been more fully discussed in Bulletin No. 30 of this Bureau. 
These investigations have shown that practically all the minerals 
originally present in rocks are found as such in arable soils. Besides 
these minerals from the original rocks, the soil naturally contains 
the products resulting from metamorphism and epigenesis, as well as 
the products resulting from synthetical processes within the soil itself. 
These minerals are all soluble to a slight degree in water, or are acted 
upon by water, i. e., they are hydrolized and the more soluble products 
of h5^drolysis are found in the soil moisture. 

The minerals present in the soil determine the character and, to a 
certain extent, the quantity of the mineral constituents in the soil solu- 
tion. The quantity of the mineral constituents in the free soil mois- 
ture is, however, largely dependent on the absorptive power of the soil, 
as will be shown in the following pages. It has long been known that 
soils have the power to absorb mineral constituents in considerable 
quantities from solution, and it is highly probable that synthetic as 
well as destructive processes are taking place in the mineral species of 
the soil. The well-known experiments of Way, Liebig, Heiden, Knop, 
Rautenberg, Peters, Yoelcker, King, and others have established the 
general fact that the arable soils show a selective absorption toward dif- 
ferent mineral constituents. In view of the importance of this subject 
to a proper understanding of the chemistry of the soil and of soil solu- 
tions, a systematic study of the behavior of several soil types toward 
phosphate and potassium has been made. The absorption of phosphate 
(POJ and of potassnum (K) has been specially studied in the light of 

7 



8 ABSORPTION OF PHOSPHATES AND POTASSIUM. 

its influence in maintaining the concentration of these two important 
plant-food constituents in the soil moisture. The rock-forming phos- 
phatic and potassium minerals of the soil yield and apparently continue 
to yield a solution whose concentration approaches equilibrium between 
the solution and the solid. As actual equilibrium is probably never 
realized under such conditions, the concentration is influenced by the 
area of surface exposed to the action of the soil moisture during a lim- 
ited period of time. This concentration appears to be constant for any 
given soil and is dependent on the nature of the minerals in the soil, 
whether they are readily acted upon by water, carbon dioxide, or other 
substances dissolved in the soil moisture, and on the absorptive capac- 
ity of the soil. The magnitude of the absorption and the slow rate of 
removal of the absorbed material from the soil make it highly proba- 
ble that the concentration of the constituents of the soil moisture is 
largely controlled b}' the absorptive power of the soil. If, for instance, 
the concentration of the soil solution in phosphate be reduced through 
any cause, such as removal of phosphate by plants or influx of rain 
water, the tendency will be to restore the original concentration by 
more of the absorbed phosphate of the soil entering into the free soil 
moisture. If, on the other hand, the phosphate content of the soil 
moisture be increased above the natural concentration for that soil — 
as, for instance, by the application of a soluble phosphatic fertilizer or 
the evaporation of soil moisture — the concentration would be reduced 
by absorption to the original strength. This is shown in the follow- 
ing experiments on the absorption of phosphate, the removal of the 
absorbed phosphate, and the removal of the phosphate originally in 
the soil. The concentration of phosphate in the solution is maintained 
with much persistence, although only a fractional part of the absorbed 
phosphate has been removed, thus indicating that while the absorbed 
phosphate is apparently rendered insoluble, it is nevertheless slowly 
but constantly going into the soil moisture. The results with potas- 
sium, though not so complete, appear to be similar in all respects to 
those obtained with the phosphate. 

DESCRIPTION OF THE SOILS. 

A preliminary series of absorption experiments was made with four- 
teen soils. They were so selected as to include types having the tex- 
ture of a clay, a clay loam, a loam, a sandy loam, and a sand. The soils 
were treated with twice their weight of a sodium phosphate (NagHPOJ 
solution containing 100 parts PO4 per million, this being equivalent to 
an application of 200 parts PO^ per million parts of the soil. After 
standing several days with occasional shaking, the supernatant liquid 
was filtered through a Pasteur-Chamberland filter and the concentra- 
tion of phosphate in the filtrate determined colorimetrically. The 
results are given in Table I. 



DESCRlPTIOl^ OF SOILS. 



Table I. — Absorption of phosphate by different soils from a solution of disodium phos- 
phate containing 100 parts PO^ per million. 



Soil. 



Cecil clay 

Susquehanna clay. . . 

Brightwood clay 

Penn loam 

Hagerstown loam . . . 

Elkton clay 

Sea island cotton soil 



Quantity 
PO4 in fil- 
trate. 



Paris per 

million. 

5 

9 

10 
5 
7 
10 
16 



Quantity 
PO4 ab- 
sorbed by 
soil. 



Parts per 
million. 
190 
182 
180 
190 
186 
180 
168 



Soil. 



Leonardtown loam 

Memphis silt loam 

Sassafras sandy loam . . . 

Cecil sandy loam 

Podunk fine sandy loam 

Norfolk fine sand 

Sandhill 



Quantity 
PO4 in fil- 
trate. 



Parts per 
million. 
28 
13 
15 
35 
25 
30 
60 



Quantity 
PO4 ab- 
sorbed by 
soil. 



Parts per 
million. 
\4A 
174 
170 
130 
150 
140 



It is at once apparent that all these soils have a marked power to 
absorb the phosphate from the solution, reducing the concentration 
from 100 to as low a concentration as 5 parts per million. On the 
whole a second treatment gave results that were much the same as in 
the jfirst, and it became obvious that the absorptive effect of the soil 
was not reduced by the phosphate already absorbed, in such quanti- 
ties as resulted from the first treatment. On the other hand, the 
different soils did show considerable variations in respect to the con- 
centration of the phosphate remaining in solution. That the absorp- 
tion is also quite rapid is shown by the following experiment, in which 
100-gram portions of the soils were treated with 500 c. c. of a mono- 
calcium phosphate (CaH^(P0j3) solution containing 100 parts per 
million PO^. This is equivalent to an application of 500 parts per 
million PO^ to the soil. At the end of the respective periods of time 
given in Table II the solutions were filtered as rapidly as possible. 
The clay mixtures required some minutes to filter, and this soil was 
therefore in contact with the solution longer than the periods indicated. 
It will be noticed that in the case of the clay soil the absorption was 
very rapid, 400 parts per million being absorbed by the soil in the 
three-minute period out of the total absorption of 445 parts per mil- 
lion in the twenty-four hour period. In the case of the fine sandy soil 
the absorption was not so rapid, only 235 parts per million being 
absorbed in the shorter period out of the 370 parts per million 
absorbed in the longer period. 

Table II. — Absorption of phosphate from a solution of monocalcium phosphate containing 

100 parts PO^ per Tnillion. 





Clay soil. 


Fine sandy soil. 


Time. 


Quantity 
P04in 

filtrate. 


Quantity 
PO4 ab- 
sorbed 
by soil. 


Quantity 
POiin 
filtrate. 


Quantity 
PO4 ab- 
sorbed 
by soil. 




Paris per 
million. 
20 
18 
17 
13 
12 
11 


Parts per 
million. 
400 
410 
415 
435 
440 
445 


Parts per 
million. 
53 
49 
48 
37 
33 
26 


Parts per 
million. 
235 




255 




260 




315 


4 hours . . . . 


335 




370 







21719— No. 32—06- 



10 



ABSOEPTION OF PHOSPHATES AND POTASSIUM. 



Four of the above soils were taken for a more thorough study, the 
selection being based on texture as well as on differences in absorp- 
tive power, as shown in the above experiment. The soils chosen were 
the Cecil clay, the Penn loam, the Podunk fine sandy loam, and the 
Norfolk fine sand. 

The following tables show the results of the mechanical and chem- 
ical analyses of the samples of these soils used: 

Table III. — Mechanical analyses of the soils used in the absorption of phosphates. 



Soil. 


Fine grav- 
el, coarse 
sand, me- 
dium sand, 
2.0 to 0.25 
mm. 


Fine sand, 
very fine 

sand, 0.25 to 
0.05 mm. 


Silt, 0.05 to 
0.005 mm. 


Clay, 0.005 
to mm. 


Cecil clay 


Per cent. 
20.3 
11.0 
2.9 
10.8 


Per cent. 
26.3 
12.8 
66.5 
69.0 


Per cent. 
22.8 
44.6 
23.1 
13.2 


Per cent. 
30 5 




31 4 


Podunk fine sandy loam 


6 8 




6 7 







Table IV. — Chemical analyses of the soils used in the absorption of phosphates, by digestion 
ivith hydrochloric acid of sp. gr. 1.115. 



Soil. 



Cecil clay 

Penn loam 

Podunk fine sandy loam 
Norfolk fine sand 



K.O. 


CaO. 


MgO. 


ALOj. 


Fe^Os. 


Per cent. 


Per cent. 


Per cent. 


Per cent. 


Per cent. 


0.19 


0.34 


0.29 


18.61 


3.28 


.61 


.32 


1.16 


11.56 


2.34 


.21 


1.37 


.85 


5.32 


1.21 


.22 


.28 


.23 


2.51 


1.19 



P205. 



Per cent. 

0.13 

.11 

.47 



The Cecil clay came from States ville, N. C, and is described'* as a 
heavy red soil from 2 to 7 inches in depth, with an average depth of 5 
inches, resting on a subsoil of stiff, tenacious red clay. It is a residual 
soil formed h^ the long-continued process of decomposition from a 
number of rock types distinct in their physical and mineralogical char- 
acters. The soil has been derived from nearly all the rocks occurring 
in this section, mainly hornblende gneiss, micaceous schists, coarse- 
grained granites, and to a less extent soapstone or steatite. Many of 
these rocks differ quite widely, yet so completely have they been 
weathered that they give rise to the same distinctive red clays. The 
change from the soil to the unweathered rock is nearly always gradual, 
and frequently it is difficult to mark the dividing line between the 
two. Often the rocks have decayed so thoroughly that it is dijBficult 
to determine their original composition. Again the structure of the 
rocks may be preserved, but on digging into what seems to be com- 
paratively sound rock the material crumbles between the fingers, show- 
ing how completely decomposition has taken place. 

« Soil Survey of the Statesville Area, North Carohna. Field Operations of the 
Bureau of Soils, 1901. 



DESCRIPTION OF THE APPARATUS. 11 

The Penn loam came from Leesburg, Va., and is described^* as con- 
sisting of from 8 to 12 inches of dark, Indian-red loam, underlain by 
a heavier loam of the same color. It is residual in origin, being 
derived from the weathering of the Triassic red sandstone. This rock 
IS composed of grains of sand coated with films of ferruginous clay. 
In the vicinity of Leesburg, Va., and northward, some wedges, or 
lenses, of limestone conglomerate are intercalated into the formation, 
thinning out very graduallj'^ into a fine point. Where this limestone 
conglomerate occurs in considerable quantities with the red sandstone 
it gives rise to the type of soil known as Penn clay. Owing to the 
very irregular and peculiar contact of the two formations the bounda- 
ries between the Penn clay and the Penn loam are not sharply marked, 
but consists of a zone of slow gradation of one type into the other. 

The sample of soil used in these experiments is shown by its mechan- 
ical analysis to be one of these gradations, having rather the texture 
of a clay loam. 

The Podunk fine sandy loam came from South Windsor, Conn., and 
is described^ as consisting of 12 inches of friable, dark-brown fine 
sandy loam, underlain with a yellow or brownish fine sandj^ loam. 
The material composing this soil originated b}^ deposition in deeper 
lake waters, but it has been largely rewashed and redeposited by later 
stream action, and much of it lies within the flood plain of the rivers 
along which it occurs. 

The Norfolk fine sand came from Upper Marlboro, Md., and is 
described^ as a light-brown or yellow loamy fine sand having a depth 
of about 8 inches. 

These four soils — the Cecil claj'^, the Penn loam, the Podunk fine 
sandy loam, and the Norfolk fine sand — were subjected to a thorough 
stud}^, both as regards the absorption of phosphate and the removal of 
the absorbed phosphate by water. This was accomplished by passing 
the solution, or water, continuously through the soil contained in a 
filter especially devised for this purpose. The rate of flow of the 
liquid through the soil was entirely under control. 

DESCRIPTION OF THE APPARATUS. 

The apparatus used is shown in figure 1. The short filter tube A is 
made by cutting off the lower part of a PasteurrChamberland filter 
tube and plugging the open end with a rubber stopper. The object in 

« Soil Survey of Leesburg Area, Virginia. Field Operations of the Bureau of Soils, 
1903. 

&Soil Survey of the Connecticut Valley. Field Operations of the Bureau of Soils, 
1899. 

cSoil Survey of Prince George County, Md. Field Operations of the Bureau of 
Soils, 1901. 



12 



ABSOEPTION OF PHOSPHATES AND POTASSIUM. 




cutting down the filter tube is twofold, first, to decrease the filtering 
surface so as to make it possible thoroughl}^ to control the flow by the 
device to be described presently, and second, to have the filtering sur- 
face as nearlj^ as practicable at the bottom of the chamber so that the 
solution must pass through the entire soil column. 
This might be more effectually accomplished by the 
use of filtering disks, but no filtering material could 
be found which was suitable for this work. The filter 
tube was cut so as to leave about 2 cm. of the tube 
projecting above the rubber stopper, which holds it 
firmly in position in the metallic tube B serving as the 
receptacle for the soil. The metallic tube is closed at 
the upper end with a two-hole rubber stopper provided 
with two glass tubes. One of these tubes is about 6 
feet in length, the end being connected with a funnel. 
If a greater pressure than 6 feet is necessary for 
forcing the liquid through a soil, a piece of tubing is 
readil}'^ added and any desired pressure obtained. A 
height of more than 6 feet was, however, necessary in 
onl}^ a single case in the course of these experiments. 
The shorter tube provided with a rubber tube and 
pinchcock serves to let the entrapped air out of the 
metal tube when starting the apparatus. 

The rate of flow of the liquid through the soil is con- 
trolled by means of the glass spiral siphon C^ con- 
structed of very thin walled capillary tubing, the 
tubing being of such a length that the friction of the 
liquid in passing through it gives the desired rate of 
flow. The spiral was suspended in the solution to be 
used, contained in a beaker of such large diameter that the level of 
the liquid was not materially changed overnight by the flow through 
the siphon. It was found that the rate of flow was not sensibly 
affected by slight changes in the height of the liquid in the reservoir. 
Devices like the Mariotte constant-level bottle were also tried, but 
the rate required was so slow that no satisfactory flow of liquid could 
be maintained. The length of the spiral was adjusted so that about 
50 c. c. passed in twenty-four hours. 

The apparatus may be charged by putting about 50 c. c. of water or of 
the solution to be used into the percolating tube and then adding the 
finely powdered soil in such a way as to prevent trapping of air, or 
the soil may be made into a thin paste with the solution and this 
poured into the tube. The latter is then set in a vertical position in a 
stand and connected with the rest of the apparatus by inserting the 



Fig. 1. — Appara- 
tus used in the ab- 
sorption e X p 6 r i - 
ments. 



REMOVAL OF PHOSPHATE BY WATER. 13 

rubber stopper. The pinchcock is opened and some of the liquid 
poured into the funnel until the metal tube is full. The pinchcock is 
then closed and the glass tube filled with liquid by pouring into the 
funnel. The constant-dropping spiral is then put in position, as shown 
in the figure, and the funnel covered to prevent evaporation. The 
pressure of the column of liquid in the tube forces the solution through 
the soil and through the filter, A, and if the rate of filtration is greater 
than the rate with which the liquid is supplied by the constant-drop- 
ping spiral the height of liquid in the glass tube will decrease until the 
pressure of the column is sufiicient to force the liquid through the 
filter at the same rate as it is supplied by the dropping siphon. As 
the column of liquid becomes shorter, the pressure is diminished and 
the filtration is less rapid, until finally the rate is exactly that of the 
dropping spiral. If the filtration should tend to become slower than 
this rate, the column of liquid in the tube will rise and the consequent 
greater pressure will be sufiicient to keep the rate of filtration constant 
and identical with that of the dropping spiral. The apparatus becomes, 
therefore, perfectly automatic as far as the rate of flow of the liquid 
is concerned. It is of course necessary that the filter be sufficiently 
close grained, a condition easily obtained with the filters mentioned, 
and that the filtering surface be sufficientl}^ small so as not to filter 
faster than the desired rate without the application of some pressure. 

REMOVAL OF PHOSPHATE BY WATER. 

A study of the absorption by soils of phosphate from aqueous solu- 
tions and the removal of the absorbed phosphate by water requires a 
definite knowledge as to the behavior of the phosphate originally in 
the soils toward the solvent action of water under the identical condi- 
tions used in the absorption experiments. To ascertain this the soils 
selected for the investigation were treated with distilled water, and 
determinations made of the phosphate in the various fractions of 
percolate. 

For this purpose the apparatus already described was used and the 
tubes were charged with 100 grams of soil. As already mentioned, 
the soils studied were a clay, a clay loam, a fine sandy loam, and a fine 
sand. The soils had been collected several months previously and were 
air dry. Pure distilled water was allowed to flow through the soil at 
the rate of 50 c. c. in twent^^-four hours. The percolate was collected 
in fractions and the phosphate concentrations determined by means of 
the colorimetric method described in a former bulletin.^ The results 
obtained with the clay soil are given in Table V. 

«Bul. No. 31, Bureau of Soils, U. S. Dept. of Agr., 1905, p. 42. 



14 



ABSORPTION OF PHOSPHATES AND POTASSIUM. 



Table V. — Removal of phosphate from a clay soil by water. 







Total 






Total 






Total 


Volume 


Quantity 


quantity 


Volume 


Quantity 


quantity 


Volume 


Quantity 


quantity 


of perco- 


of PO4 in 


of PO4 ex- 


of perco- 


of PO4 in 


of PO4 ex- 


of perco- 


of P04in 


of PO4 ex- 


late. 


solution. 


tracted 
from soil. 


late. 


solution. 


tracted 
from soil. 


late. 


solution. 


tracted 
from soil. 


Cubic cen- 


Parts per 


Parts -per 


Cubic cen- 


Parts per 


Parts per 


Cubic cen- 


Parts per 


Parts per 


timeters. 


million. 


million. 


timeters. 


million. 


million. 


timeters. 


million. 


million. 


40 


26 


10 


360 


7 


35 


690 


7 


55 


130 


8 


18 


540 


6 


45 


740 


5 


57 


170 


8 


21 


590 


6 


48 


790 


7 


61 


260 


8 


28 


640 


6 


61 


850 


7 


64 



In the first column is given the total number of cubic centimeters 
of the solution which have passed through the soil, the concentrations 
of the separate fractions in phosphate being given in the second col- 
umn. In the third column are the figures for the total quantity- 
removed by the water, the results being expressed in terms of parts 
PO4 per million parts of the soil. It will be noticed that after the first 
portion, which is considerably stronger than the others, has passed 
the solution has practically a constant concentration in phosphate. 

The results for the clay loam are given in Table VI. In this case 
the concentration is likewise greatest in the first portion, diminish- 
ing with succeeding portions, although the drop is not so abrupt as 
with the clay soil, until again a constant concentration of solution is 
obtained. 

Table VI. — Removal of phosphate from a day loam hy water. 







Total 






Total 






Total 


Volume 


Quantity 


quantity 


Volume 


Quantity 


quantity 


Volume 


Quantity 


quantity 


of perco- 


of PO4 in 


of PO4 ex- 


of perco- 


of PO4 in 


of PO4 ex- 


of perco- 


of PO4 in 


of PO4 ex- 


late. 


solution. 


tracted 
from soil. 


late. 


solution. 


tracted 
from soil. 


late. 


solution. 


tracted 
from soil. 


Cubic cen- 


Parts per 


Parts per 


Cubic cen- 


Paris per 


Parts per 


Cubic cen- 


Parts per 


Paris per 


timeters. 


million. 


million. 


timeters. 


million. 


million. 


timeters. 


million. 


million. 


30 


19 


6 


250 


6 


28 


510 


6 


45 


80 


17 


15 


360 


6 


35 


550 


6 


47 


140 


10 


20 


410 


6 


38 








190 


10 


25 


460 


7 


41 









In Table VII are given the results obtained with the fine sandy loam. 
Here the first result is lower than the succeeding ones, but likewise 
there is the same tendency to yield a solution of constant concentra- 
tion in phosphate, although this is considerably higher than in the 
solutions obtained from the other soils. This is perhaps due to the 
greater rate of solubility of the phosphatic mineral constituents" of 
the Podunk fine sandy loam used in this experiment as well as to the 
lower absorptive capacity of this soil. The other minerals of this soil 
are likewise quite soluble, and a solution quite high in total salts, com- 
paratively speaking, is obtained. 

«Cameron and Hurst [Jour. Amer. Chem. Soc, 26, 885 (1904)] have shown that 
for short-time intervals the rate of solubiUty of phosphoric acid from slightly soluble 
phosphates is conditioned by the ratio between the solid and the water. 



SOLUTION" OP MONOCALCIUM PHOSPHATE. 
Table VII. — Removal of phosphate from a fine sandy loam by water. 



15 



Volume 
of perco- 
late. 


Quantity 
of PO4 in 
solution. 


Total 
quantity 
of PO4 ex- 
tracted 
from soil. 


Volume 
of perco- 
late. 


Quantity 
of PO4 in 
solution. 


Total 
quantity 
of PO4 ex- 
tracted 
from soil. 


Volume 
of perco- 
late. 


Quantity 
of PO4 in 
solution. 


Total 
quantity 
of PO4 ex- 
tracted 
from soil. 


Cubic cen- 
timeters. 
50 
100 
150 


Parts per 

million. 

17 

25 

25 


Parts per 

million. 

8 

22 

34 


Cubic cen- 
timeters. 
200 
300 
350 


Parts per 

million. 

23 

21 

18 


Parts per 

million. 

45 

55 

64 


Cubic cen- 
timeters. 
400 
460 
510 


Parts per 

million. 

19 

19 

21 


Parts per 

million. 

75 

85 

96 



The results obtained with the fine sandy soil are given in Table VIII 
and are similar to those already given. 

Table YIII. — Removal of phosphate from a fine sandy soil by water. 



Volume 
of perco- 
late. 


Quantity 
of PO4 in 
solution. 


Total 
quantity 
of PO4 ex- 
tracted 
from soil. 


Volume 
of perco- 
late. 


Quantity 
of POiin 
solution. 


Total 
quantity 
of PO4 ex- 
tracted 
from soil. 


Volume 
of perco- 
late. 


Quantity 
of P04in 
solution. 


Total 
quantity 
of PO4 ex- 
tracted 
from soil. 


Cc. 

60 

110 

180 

230 


P. p.m. 
16 
12 

7 

7 


P. p.m. 
9 

36 
21 
25 


Cc. 
380 
430 
480 
530 


P. p. m. 
5 
7 
6 
5 


P. p.m. 
32 
36 
39 
42 


Cc. 
580 
640 


P. p.m. 
6 
6 


P. p.m. 
45 

48 



The general tendency of the soils is to yield more concentrated 
fractions at the start, but soon to give a solution of nearly constant 
concentration in phosphate for each soil examined. The higher 
concentration obtained at the start is interesting, especially in the 
light of some of the absorption phenomena to be described further on 
in this bulletin. To ascribe this to the existence in the soil of more 
readily soluble phosphates which are quickly leached out seems not 
to be tenable in view of the great absorptive power of these soils for 
phosphate, even when introduced in exceedingly soluble forms, as 
has already been shown. The greater concentration in the first por- 
tions seems rather to be connected with the air-dry condition of the 
soils. In earlier bulletins from this Bureau '^ it has been demonstrated 
that oven and air dried soils yield a greater quantity of soluble salt to 
water than do the same soils in the moist condition. This greater 
concentration in phosphate in the case of the dried soils may be, at 
least in part, due to the lower absorptive power of the soil when used 
in the dry form, and will be considered later. 



ABSORPTION OF PHOSPHATE FROM A SOLUTION OF MONOCALCIUM 

PHOSPHATE. 

The phosphate used in these experiments was the monocalcium phos- 
phate, CaH^(P0j2, as this is the most soluble of the phosphates of 

« Bureau of Soils, U. S. Dept. of Agr., BuL 22, 1903, p. 42; Bui. 26, 1905, p. 55. 



16 



ABSORPTION OF PHOSPHATES AISTD POTASSIUM. 



calcium and also the one occurring in superphosphate fertilizers. A 
solution containing 200 parts of PO^ per million of the solution was 
prepared by diluting a stronger solution which had been standardized 
by gravimetric analysis. 

In these experiments the same samples of soils used in the previous 
experiment with the water percolation were studied. The tubes of soil 
were allowed to drain thoroughly. The apparatus was then filled with 
this solution as already described under the water percolation. The 
volume of water or rather soil solution remaining in the apparatus 
must necessarily have been less than 50 e.c. and this volume of perco- 
late was discarded at the beginning of the flow of solution through it. 
The flow was maintained at a rate of about 50 e.c. in twenty-four 
hours. Fractional percolates were collected and the phosphate deter- 
mined colorimetrically. The results obtained with the clay soil are 
given in Table IX. In the first column is given the total volume of 
the phosphate solution which has passed through the soil, the second 
column giving the concentration of phosphate in the separate portions. 
The third column gives the amount of phosphate absorbed by the soil, 
expressed in parts per million. In the fourth column are given the 
results calculated by means of a formula derived from certain theoret- 
ical considerations which will be developed later. 

It will be seen from the table that the phosphate is almost completely 
absorbed from the first ■iOO e.c. passed, the concentration having been 
reduced from 200 parts per million to 6 and 8 parts per million, practi- 
call}^ the concentration juelded by the percolation of pure water through 
the soil. As more solution flows through the soil the concentration 
of phosphate in the percolate increases, rapidly at first, and then more 
slowly. The absorption is still going on to a considerable extent even 
after the passage of nearly 6 liters of the solution and a total absorp- 
tion by the soil of nearl}^ 5,000 parts per million of phosphate. The 
quantity absorbed by the soil as shown in the third column increases 
rapidly at first and then more slowly as the total amount already 
absorbed increases. The fourth column will be discussed presently. 

Table IX. — Absorption of phosphate by a clay soil from a solution of monocalcium phos- 
phate, CaH^{P0^2i containing 200 jiarts pex million PO^. 







Total quantity 






Total quantity 






Total quantity 


Vol- 


Quan- 


PO4 absorbed 


Vol- 


Quan- 


PO4 absorbed 


Vol- 


Quan- 


PO4 absorbed 


ume of 


tity PO4 
in solu- 


by soil. 


ume of 


tity PO4 
in solu- 


by soil. 


ume of 


tity PO4 
in solu- 


by soil. 




















late. 


tion. 


Ob- 


Calcu- 


late. 


tion. 


Ob- 


Calcu- 


late. 


tion. 


Ob- 


Calcu- 






served. 


lated. 






served. 


lated. 






served. 


lated. 


e.c. 


P. p.m. 


P. p.m. 


P. p.m. 


e.c. 


P. p.m. 


P. p.m. 


P. p. m. 


e.c. 


P. p.m. 


P. p.m. 


P. p.m. 


250 


6 


480 


480 


1,660 


101 


2,470 


2,480 


3,890 


150 


4,100 


4,160 


410 


8 


790 


740 


2,050 


115 


2,800 


2,880 


4,290 


152 


4,300 


4,330 


570 


12 


1,090 


1,020 


2, ,510 


108 


3,220 


3,270 


4,640 


156 


4,460 


4,480 


780 


31 


1,4.50 


1,3.50 


2,820 


111 


3.440 


3, 510 


5,000 


162 


4, .590 


4,600 


1,030 


58 


1,800 


1,720 


3,260 


127 


3,760 


3,810 


5,370 


158 


4,740 


4,720 


1,330 


84 


2, 1.50 


2, 100 


3,640 


142 


3,980 


4,030 


5,740 


169 


4,860 


4,820 



SOLUTION OF MONOCALCIUM PHOSPHATE. 



17 



In Table X are found the results obtained with the clay loam. It 
will be noticed that here again the concentration of the first portions 
is very low, approximately that obtained by the action of water on 
the soil, and that the concentration of the percolate rises as more solu- 
tion is passed through the soil, but much more rapidly than in the 
case of the clay soil. The concentration when about 4 liters of solu- 
tion have passed through the clay loam is appreciably^ greater than 
when 6 liters of solution have passed through the clay soil. The 
quantity absorbed hy the clay loam is considerably less than by the 
clay soil when any given volume of solution has passed through the 
soil. The column giving the quantities absorbed by the clay loam is 
not materially different from that in the table for the clsiy soil, except 
in the quantities absorbed. This will be discussed later with the results 
in the last column. 



Table X. — Absorption of phosphate by a day loam from a solution of monocalcium 
phosphate, CaH^{P0^).2, containing SOO parts per million PO^. 







Total quantit}^ 






Total quantity 






Total quantity 


Vol- 


Quan- 


PO4 absorbed 


Vol- 


Quan- 


PO4 absorbed 


Vol- 


Quan- 


PO4 absorbed 


ume of 


tity PO4 
in solu- 


by soil. 


ume of 


tity PO4 
in solu- 


by soil. 


ume of 


tity PO4 
in solu- 


by soil. 




















late. 


tion. 


Ob- 


Calcu- 


late. 


tion. 


Ob- 


Calcu- 


late. 


tion. 


Ob- 


Calcu- 






served. 


lated. 






served. 


lated. 






served. 


lated. 


C.c. 


P. p. m. 


P. p. m. 


P. p. m. 


C.c. 


P. p. m. 


P. p. m. 


P. p. m. 


C.e. 


P. p. m. 


P. p. m. 


P. p. m. 


110 


6 


220 


200 


1,420 


138 


1,830 


1,810 


3,410 


167 


2,720 


2, 720 


220 


6 


420 


390 


1,690 


137 


2,000 


2,000 


3,700 


176 


2,790 


2,780 


360 


38 


650 


610 


1,890 


159 


2,080 


2,130 


4,050 


187 


2,880 


2,850 


590 


60 


990 


940 : 


2,170 


155 


2,210 


2, 280 


4,340 


189 


2,860 


2,900 


730 


47 


1,200 


1,120 


2,470 


150 


2,360 


2,420 


4,570 


178 


2,920 


2,910 


940 


78 


1,460 


1,370 


2, 800 


148 


2,530 


2, 550 










1,110 


95 


1,640 


1,550 


3,150 


170 


2,630 


2, 650 











In Table XI will be found the results obtained with the fine sandy 
loam. These are very similar in character to those for the soils 
already given, the concentration of the first fraction being again 
approximately that of the solution obtained from the soil by percola- 
tion with pure water. The total amount absorbed by the soil is, 
moreover, less throughout than that absorbed by either the clay loam 
or clay soil. 

Table XI. — Absorp)tion of phosphate by a fine sandy loam from a solution of monocalcium 
phosphate, Call^^POi)^, containing 200 parts per million PO^. 







Total quantity 1 






Total quantity 






Total quantitv 


Vol- 


Quan- 


PO4 absorbed 


Vol- 


Quan- 


PO4 absorbed 


Vol- 


Quan- 


PO4 absorbed 


ume of 


tity POi 
in solu- 


by soil. 


ume of 


tity PO4 
in solu- 


by soil. 


ume of 


tity PO4 
in solu- 


by soil. 




















late. 


tion. 


Ob- 


Calcu- 


late. 


tion. 


Ob- 


Calcu- 


late. 


tion. 


Ob- 


Calo<i- 






served. 


lated. 






served. 


lated. 






served. 


lated. 


C.c. 


P. p.m. 


P. p.m. 


P. p.m.' 


C.c. 


P. p.m. 


P. p. m,. 


P. p.m. 


C.c. 


P. p.m. 


P. p.m. 


P. p.m. 


150 


22 


260 


290 


1,270 


127 


1,660 


1,650 


3,000 


156 


2, 460 


2,450'. 


280 


27 


500 


500 1 


1,450 


146 


1,7.50 


1,780 


3, 300 


185 


2,500 


2,510 


430 


31 


740 


740 ■ 


1,660 


135 


1,890 


1,920 


3,540 


190 


2,530 


2,560 


640 


55 


1,040 


1,010 ' 


1,970 


157 


2,020 


2,080 


3,860 


177 


2,600 


2,610 


840 


64 


1,320 


1,2.50 


2, 340 


167 


2,140 


2,250 


4,150 


185 


2, 640 


2,640 


1,040 


115 


1,490 


1,450 


2,710 


149 


2,330 


2,370 


4,390 


182 


2,690 


2, 670 



21719— No. 32—06- 



18 



ABSORPTION OF PHOSPHATES AND POTASSIUM. 



In Table XII are given the results obtained with the tine sandy soil. 
The absorptive power of this soil is considerably less than that of the 
soils alread}^ considered, as is shown by the third column of figures 
and also by the relatively higher concentrations in the phosphate solu- 
tions throughout. It will be noticed also that this lower absorptive 
power of the soil is shown by the fact that the concentration of the 
first fractions is higher than the concentration obtained by percolating 
water through the soil, while with the other soils they were approxi- 
mately the same. 

Table XII. — Absorption of phosphate by a fine sandy soil from a solution of monocnlcium 
phosphate, CaH^{POi).,, cordaining 200 jyarts per million PO^. 







Total quantity 






Total quantity 






Total quantity 


Vol- 


Quan- 


PO4 absorbed 


Vol- 


Quan- 


PO4 absorbed 


Vol- 


Quan- 


PO4 absorbed 


vim e of 


tity FO4 
in solu- 


by soil. 


ume of 


tity PO4 
in solu- 


by soil. 


ume of 


tity PO4 
in solu- 


by soil. 


















late. 


tion. 


Ob- 


Calcu- 


late. 


tion. 


Ob- 


Calcu- 


late. 


tion. 


Ob- 


Calcu- 






seryed. 


lated. 






?e;yed. 


lated. 






served. 


lated. 


C.c. 


P. p. m. 


P. p. m. 


P. p. in. 


C.c. 


P. p. m. 


P. p m. 


P. p. m. 


a c. 


P. p. m. 


P. p. m. 


P. p. in. 


140 


20 


250 


210 


1,150 


152 


1,210 


1,260 


2,750 


159 


2, 050 


2,060 


270 


19 


480 


400 


1,340 


141 


1,320 


1,400 


3,090 


148 


2,230 


2,150 


380 


39 


660 


530 


1,560 


142 


1,440 


1,540 


3,330 


174 


2,290 


2,210 


540 


82 


850 


700 


1,800 


159 


1,540 


1,660 


3, 620 


191 


2,320 


2, 270 


670 


121 


950 


850 


1,980 


122 


1,690 


1,760 


3,850 


192 


2,340 


2,310 


810 


129 


1,050 


980 


2,210 


143 


1,820 1 1,870 


4,040 


191 


2,350 


2,340 


950 


156 


1,120 


1,100 


2,460 


155 


1,930 1,960 




1 





REMOVAL OF ABSORBED PHOSPHATE BY WATER. 

At the conclusion of the absorption experiments described in the 
preceding section the tubes with the soils were allowed to drain 
thoroughly, and then the apparatus was filled with distilled water in 
the manner already described. The flow of water was again at the 
rate of about 50 c. c. in twenty-four hours, the percolate being col- 
lected in fractions and the concentration of phosphate determined 
colorimetrically as before. The results obtained with the ck}" soil are 
given in Table XIII. The first column gives the volume of solution 
which has passed through the soil and the second the concentrations of 
the separate fractions as before. At the head of the third column is 
given the total quantity of phosphate absorbed by the soil in the pre- 
vious experiment, and following this are the amounts of absorbed 
phosphate not washed out. 

Table XIII. — Removal of absorbed phosphate from a clay soil by tvater. 



Vol- 
ume of 
perco- 
late. 


Quan- 
tity 

P04in 
solu- 
tion. 


Total 
quan- 
tity 
PO4 re- 
main- 
ing in 
soil. 


Vol- 
ume of 
perco- 
late. 


Quan- 
tity 

P04in 
solu- 
tion. 


Total 
quan- 
tity 
PO4 re- 
main- 
ing in 
soil. 


Vol- 
ume of 
perco- 
late. 


Quan- 
tity 

P04in 
solu- 
tion. 


Total 
quan- 
tity 
PO4 re- 
main- 
ing in 
soil. 


Vol- 
ume of 
perco- 
late. 


Quan- 
tity 

P04in 
solu- 
tion. 


Total 
quan- 
tity 
PO4 re- 
main- 
ing in 
soil. 


C.c. 


P. p.m. 


P. p.m. 
4,860 
4,730 
4,630 
4,510 
4,450 


C.c. 
730 
910 
1,140 
1,330 
1,750 


P. p.m. 
36 
32 
27 
■ 21 
21 


P. p.m. 
4,410 
4,350 
4,290 
4,250 
4,160 


C.c. 
2,280 

2, 590 
2,970 
3,370 
3,870 


P.p.m. 
20 
19 
16 
16 
11 


P.p.m. 
4,050 
3,990 
3,930 
3, 870 
3,810 


C.c. 

4,450 

5,030 

5,450 

5,870 

6,300 


P.p.m. 
8 

7 
8 
7 
7 


P.p. m. 

3,760 

3,720 

3,690 

3,660 

3,630 


100 
230 
440 
580 


127 
85 
55 
41 



EEMOVAL OF ABSORBED PHOSPHATE BY WATEE. 



19 



It* will be noticed that the concentration of the first fractions is 
very hig'h. The concentration in phosphate decreases rapidly at first 
and then very slowly, until at about 4 liters the concentration of the 
solution has become constant and is practically the same as that found 
at the beginning of the absorption and that of the percolate from the 
original soil, although the quantity of absorbed phosphate still remain- 
ing in the soil is nearl}^ 3,800 parts per million, or approximatel}^ 75 
per cent of the total phosphate absorbed. The results with the clay 
loam, given in Table XIV, show a similar tendency. The high con- 
centration in phosphate in the first portions gradually gives place to 
lower concentrations, until at about 5 liters the concentration reached 
is that of the solution obtained at the start of the absorption or by 
percolating water through the original soil. The amount of absorbed 
phosphate still in the soil is, however, considerable at this point, 
being about 1,900 parts per million, or approximate!}^ 65 per cent of 
the total phosphate absorbed. 

Table XIV. — Removal of absorbed phosphate from a clay loam bi/ water. 



Vol- 
ume of 
perco- 
late. 


Quan- 
tity PO4 
in solu- 
tion. 


Total i 
quan- 
tity 
PO4 re- 
main- 
ing in 
soil. 


Vol- 
ume of 
perco- 
late. 


Quan- 
tity PO4 
in solu- 
tion. 


Total 
quan- 
tity 
PO4 re- 
main- 
ing in 
soil. 


Vol- 
ume of 
perco- 
late. 


Quan- 
tity PO4 
in solu- 
tion. 


Total 
quan- 
tity 
PO4 re- 
main- 
ing in 
soil. 


Vol- 
ume of 
perco- 
late. 


Quan- 
tity PO4 
in solu- 
tion. 


Total 
quan- 
tity 
PO4 re- 
main- 
ing in 
soil. 


C.c. 


P. p.m. 


P. p.m. 
2,920 
2,840 
2,760 : 
2,670 ; 
2,690 

■ 2,540 ' 


C.c. 

1,030 

1,250 

1,460 

1,700 

1,900 

2,200 


P.p. m. 
23 

17 
17 
16 
15 
14 


P.p. m. 
2,500 
2,460 
2,420 
2,390 
2, 360 
2, 310 


C.c. 

2,460 

2,770 

3,080 

3,400 

3,730 

3,970 


P. p.m. 
17 
17 
13 

16 
15 
14 


P. p.m. 
2,270 
2,210 
2,180 
2,120 
2,070 
2,040 


ac. 

4, 530 
5,100 
5,690 
6,270 
6,830 
7,340 


P. p.m. 
11 
8 
7 
7 
6 
6 


P. p.m. 
1,980 
1,940 
1,900 
1,860 
1,820 
1,790 


90 
180 
400 
630 
830 


89 
77 
41 
36 
25 



The results obtained with the fine sandy loam are given in Table 
XY. A concentration of phosphate in the solution approximately 
equal to that of the first fraction in the absorption experiments, or to 
that of the solution obtained by percolating water through the original 
soil, is found to be reached when about 3 liters of percolate have been 
obtained. The absorbed phosphate still remaining in the soil at this 
point is about 1,600 parts per million, or about 60 per cent of the total 
quantity absorbed. The concentration of the solution in phosphate 
decreases, however, until it falls considerably below that of the solu- 
tions obtained by percolating water through the original soil, although 
there is present a much larger amount of phosphate in the soil at this 
stage than there was in the original soil. 



20 



ABSORPTION OF PHOSPHATES AND POTASSIITM. 



Table XV. — Removal of absorbed phosphate from a fine sandy loam by imter. 



Vol- 
ume of 
perco- 
late. 


Quan- 
tity 

P04in 
solu- 
tion. 


Total 
quan- 
tity 
PO4 re- 
main- 
ing in 
soil. 


Vol- 
ume of 
perco- 
late. 


Quan- 
tity 

P04in 
solu- 
tion. 


Total 
quan- 
tity 
PO4 re- 
main- 
ing in 
soil. 


Vol- 
ume of 
perco- 
late. 


Quan- 
tity 

P04in 
solu- 
tion. 


Total 
quan- 
tity 
PO4 re- 
main- 
ing in 
soil. 


Vol- 
ume of 
perco- 
late. 


Quan- 
tity 

PO4 in 
solu- 
tion. 


Total 
quan- 
tity 
PO4 re- 
main- 
ing in 
soil. 


C.c. 


P. p.m. 


P. p.m. 
2,690 
2.540 
2,430 
2,330 
2,200 
2,110 


C.c. 

1,030 

1,240 

1,470 

1,690 

1,890 

2,120 


P. p.m. 
34 
30 
28 
26 
23 
22 


P. p.m. 
2,040 
1,980 
1,910 
1,860 
1,810 
1,760 


C.c. 

2,320 

2,820 

3,140 

3,490 

3,890 

4,290 


P. p. III. 
21 
23 
23 
22 
15 
13 


P. p. m. 
1,720 
1,600 
1, 530 
1,450 
1,390 
1,340 


ac. 

4,600 
5,180 
5,720 
6,140 
6,570 


P. p. m. 
12 
11 
10 
11 
10 


P. p.m. 

1,300 

1,240 

1,190 

1,140 

1,100 


i25 

250 
3S0 
610 
820 


124 
87 
76 
56 
42 



It has been pointed out alread}- that the Podunk fine sandy loam 
used in these experiments contained minerals which were acted upon 
by water to a very considerable extent. The percolation of over 40 
times its weight of the calcium phosphate solution, containing- 42 parts 
per million Ca in addition to the 200 parts per million PO^, together 
with the subsequent washing with over 60 times its weight of water, 
has produced chemical changes in the soil itself, so that we have no 
longer the identical soil started with. In fact, this must be the con- 
clusion reached from the figures given above, so far as the removal of 
the phosphate from the soil is concerned. In Table XVI are found 
the results for the fine sandy soil. 

Table XVI. — Removal of absorbed pihosphate from a fine sandy soil by water. 



Vol- 
ume of 
perco- 
late. 


Quan- 
tity 

PO4 in 
solu- 
tion. 


Total 
quan- 
tity 
PO4 re- 
main- 
ing in 
soil. 


Vol- 
ume of 
perco- 
late. 


Quan- 
tity 

P04in 
solu- 
tion. 


Total 
quan- 
tity 
PO4 re- 
main- 
ing in 
soil. 


Vol- 
ume of 
perco- 
late. 


Quan- 
tity 

P04in 
solu- 
tion. 


Total 
quan- 
tity 
PO4 re- 
main- 
ing in 
soil. 


Vol. 
umeof 
perco- 
late. 


Quan- 
tity 

P04in 
solu- 
tion. 


Total 
quan- 
tity 
P04 re- 
main- 
ing in 
soil. 


C.c. 


P.p. m. 


P. p. m. 
2, .3.50 
2,19C 
2,120 
2,060 
2,020 
1,970 


C.e. 
1,060 
1,270 
1,490 
1,740 
1,950 
2,180 


P. p. m. 
19 
17 
13 
13 
12 
11 


P.p. m. 
1,930 
1,890 
1,860 
1,830 
1,810 
1,780 


Cc. 
2,400 
2,680 
2,940 
3, 200 
3,540 
4,100 


P. p. m. 
10 
9 
8 

9 ■ 
7 
8 


P.p. m. 
1,760 
1,740 
1,710 
1,690 
1,670 
1, 620 


a c. 
4,610 
5,090 
5,670 
6,230 
6,660 
7,080 


P. p. m. 
7 
7 
5 
6 
6 
5 


P. p. m. 
1,590 
1,550 
1,530 
1,490 
1,470 
1,450 


110 
240 
430 
620 
840 


150 
51 
31 
24 
21 



The concentration of the phosphate in the fractions of percolate 
decreases rapidly at first and then very slowly, approaching a uniform 
concentration toward the end, which is considerably lower than that of 
the first fraction obtained in the absorption experiment. It is approxi- 
matel}^ the concentration obtained from the original soil hj passing- 
pure water through it, although the amount of phosphate present in 
the soil is considerabh" greater. 



RESULTS WITH MONOCALCIUM PHOSPHATE. 21 

GRAPHICAL REPRESENTATION AND DISCUSSION OF THE RESULTS OBTAINED 
WITH MONOCALCIUM PHOSPHATE. 

The results obtained in the foregoing experiments on the absorption 
of phosphate from a solution of monocalcium phosphate, together with 
those obtained in removing the absorbed phosphate, are shown graphic- 
ally in the following figures. In figure 2 are shown the results 
expressed in terms of the solutions. The abscissas represent the liters 
of solution or of water which have been passed through the 100 grams 
of soil in the percolating tube. The ordinates give the concentration of 
the percolate in phosphate. The upper boundar}^ line of the figure 
represents the strength of the solution before passing through the 
soil; the break in each of the curves gives the point where the 200 
parts per million solution was replaced by distilled water and the 
washing out of the absorbed phosphate was begun. The intersection 
of each of the curves on the axis of ordinates shows the concentration 
of phosphate solution obtained by percolating water through the origi- 
nal soil. The curves in this figure are smoothed curves; the experi- 
mental points are not given in the diagram, since the large number 
and intermingling of the points would be confusing, as can be seen by 
examining the tables. The curves, however, represent very well the 
general facts brought out by the figures in the tables. It will be noticed 
that the absorption of phosphate from the solution by the soil is consider- 
able for approximately the first 300 c. c. of the solution passed, and 
that the concentration of the solution is practically reduced to that of 
the soil extract obtained from the original soil. After this the curves 
rise with increasing volume of solution passed, rapidly in the case of 
the sandy soil and much more slowly in the case of the clay soil. All 
the curves approach the line representing the concentration of the 
original solution, namel}", 200 parts per million, apparently as an 
asymptote. The curves for the removal of the absorbed phosphate 
show a very decided drop at the start and then rapidly tend to become 
horizontal when concentrations have been reached approximating those 
of the water extracts of the original soils or those of the first few 
fractions of percolate in the absorption experiments. This is shown 
by comparing the extremes of the curves. It is, moreover, significant 
that the soils, after this absorption treatment, yield solutions which 
are much closer together in concentration of phosphate than did the 
original soils, although the quantities of phosphate in the soils are in 
all cases greater than those originally present. The concentrations at 
the ends of the curves are practically identical in the case of the fine 
sandy soil, clay loam, and clay soil studied. In the case of the fine 
sandy loam the concentration of the aqueous extract of the original 
soil is somewhat greater than the concentration of the last percolate, 



22 



ABSORPTION OF PHOSPHATES AND POTASSIUM. 



although the soil actually contains more phosphates than it did 
originall3\ 

In figure 3 are shown the results on the basis of the soil itself, the 
abscissas being the liters of solution passed as before and the ordi nates 
the parts of phosphate per million parts of soil. The experimental 
points as found in the tables are indicated in the figure by specific 
signs. It is at once apparent that the absorption curves of the differ- 




FiG. 2. — Solution curves. Absorption of phosphate by soils from a solutionof monocalcinm phosphate 
and the removal of the absorbed phosphate'by water. . 



CLftY^SOIL 




PKt. 3. — Soil curves. Absorption of phosphate by soils from a solution of monocalcium phosphate and 
the removal of the absorbed phosphate by water. 

ent soils are quite similar in character, although these soils diflfer in the 
amount of the phqsphates absorbed at any given abscissa. The curve 
for the sand}^ «oil indicates a rapid approach to a horizontal position, 
that is, a condition of saturation for phosphate, at about 2,400 parts 
per million, whereas the curve for the clay soil at about twice the height 
is still showing a decided tendency to rise. The curves for the removal 
of the absorbed phosphate drop rapidly at first and then run smoothly 
and graduall}^ downward. The removal curve for the sandy loam has 
a steeper slope than the others, which fact is of course also shown by 
the higher concentration of the phosphate solution, as appears from 
figure 2. A comparison of the two figures shows also very strikingl}^ 
that with all the soils studied a solution of low phosphate content, 
approximately of the strength of the extract obtained from the origi- 
nal soil, is obtained when only a fractional part of the absorbed phos- 
phate has been removed. 



RESULTS WITH MONOCALCIUM PHOSPHATE. 23 

These graphical representations of the absorption results indicate, 
as has already been pointed out, that the soils are approaching a satu- 
ration for phosphate under the conditions of the experiment, as is 
shown by the fact that each curve is evidentl}^ approaching a horizon- 
tal asymptote. It has been found that these absorption phenomena 
are quite accuratel}" represented b}^ an expression of the same form as 
monomolecular reaction velocities, rate of solution, and other analogous 
processes. Lagergren" has already shown that the rate of adsorption 
of several organic acids by charcoal, in respect to time, behaves like 
a monomolecular reaction. 

If we represent the maximum quantity that the soil can take up from 
the 200 parts per million solution, that is, the ordinate of the asymptote 
just mentioned, and let y equal the parts per million PO^ it has taken 
up when the volume v has passed through the soil, the simplest assump- 
tion that can be made is that the quantity absorbed from a unit volume 
of solution as it passes through the soil is proportional to the amount 
the soil can still take up. This is represented by the following differ- 
ential equation: 

Integrating between limits 




we get 

log (^4 — «'/)— log A^—Kv, 
or 

log {A—y) =log A~Kv. 

Since log A and K are constants to be found from the observations, 
Briggsian instead of Naperian logarithms may be used in the calcula- 
tions. This will merely change the values of A^'and log A in the same 
ratio. 

The simplest way to test the applicability of the equation appears 
to be (1) to determine A by inspection of the curve obtained b}" plot- 
ting the experimental data; (2) to make a table of log {A—y)\ (3) to 
make a plot using v as abscissa and log {A~y) as ordinate. It will 



«Bihang till K. Sv. Vet.-Akad. Handl., 24, Afd. II, No. 5 (1898). 



24 



ABSOKPTTOlSr OF PHOSPHATES AND POTASSIUM. 



be n&ticed that the above logarithmic equation is that of a straight line. 
The plot will therefore be a straight line if the correct value for J_ 
has been chosen. The intercept on the axis of ordinates is log A and 
the slope of the line gives the constant Ii. 

The absorption results for all of the soils studied have been tested 
in this way, and it has been found that a fair agreement with the 
formula is obtained when the following constants are used: 



Soil. 


A. 


log^. 


K. 




5, 500 
3,100 
2,800 
2,600 


3.740 
3.491 
3.447 
3.413 


0. 000158 
. 000267 
. 000299 
. 000247 






Fine sandy soil 





The results obtained by using these values in calculating the parts per 
million PO^ absorbed b}^ the soil for any given volume of the solution 
which has percolated through it are given in the fourth columns of 
the tables for the respective soils. In figure 3 the absorption curves 
are drawn through the calculated points, and the experimental points 
are indicated by the specific signs. The plots show, therefore, how 
well the calculated and. the experimental results agree. These are 
given in the third and fourth columns, respectively, of the absorption 
tables. 

Apparently the removal curves are likewise represented by a similar 
formula when the first few points are left out of consideration. This 
disagreement for the first few points may be due to the fact that a part 
of the strong phosphate solution remained in the soil when the wash- 
ing process was begun. The equation for the removal of phosphates 
from the soil may be written as follows: 



i=^^'0/- 



i?) 



where Jj is the ordinate corresponding to the asymptote of the removal 
curve — that is, the amount of phosphate which has apparently become 
permanently insoluble so far as any reasonable amount of solvent pass- 
ing through it is concerned. The calculated results obtained with this 
equation bring out no information not given b}^ the plotted curves, 
and are therefore omitted. 



ABSORPTION OF PHOSPHATE FROM A SOLUTION OF DISODIUM PHOSPHATE 
AND REMOVAL OF THE ABSORBED PHOSPHATE BY WATER. 

In the following experiments a solution of sodium phosphate was 
used instead of the monocalcium phosphate of the previous experi- 
ments. The solution used contained 200 parts of PO^ per million of 
the solution and was prepared by diluting a strong solution which had 



DISODIUM PHOSPHATE SOLUTION'. 



25 



been standardized by gravimetric analysis. The same clay, clay loam, 
fine sandy loam, and fine sand were used as in the experiments with 
calcium phosphate. The apparatus was charged by putting some of 
the solution into the percolating tube and then adding the dry soil. 
The tube was then completely filled with solution in the manner 
already described, and all other operations were exactly the same as in 
the preceding experiments. The absorption was not carried so far as 
in the experiments with the calcium phosphate. 

Table XVII. — Absorption of phosphate by a day soil from a solution of sodium phosphate, 
Na^HPO^, containing 200 parts per million PO^. 



Volume 
of perco- 
late. 


Quantity 
PO4 in so- 
lution. 


Total quantity PO4 
absorbed by soil. 


Volume 
of perco- 
late. 


Quantity 
PO^ in so- 
lution. 


Total quantity PO4 
absorbed by soil. 


Ob- 
served. 


Calcu- 
lated. 


Ob- 
served. 


Calcu- 
lated. 


C.c. 
190 
220 
320 
410 
520 


P. p. m. 
154 
23 

8 
5 
8 


P. p. m. 
90 
150 
330 
510 
710 


P. p. m. 


C.c. 
610 
740 
850 
900 
970 


P. p. m. 
12 
24 

42 
48 
61 


P. p. m. 
890 
1,110 
1,280 
1,360 
1,470 


P. p. m. 
900 
1,110 
1,280 
1,370 
1,480 


150 
330 
510 
710 



In Table XVII are given the absorption results with the clay soil. 
As before, the fourth column gives the amount calculated by a for- 
mula to be discussed presently. It is at once apparent from the 
second column, giving the concentration in phosphate of the separate 
percolates, that the results differ from those where the wet soil was 
treated with the solution of monocalcium phosphate. The concentra- 
tion in phosphate, instead of being low at the very start and then 
gradually rising, is in this case quite high at first, rapidly drops down 
to a very low concentration, and then rises gradually. This high con- 
centration at the start may be due in part to the more rapid movement 
of the liquid through the soil in the beginning, while the rate of flow 
is becoming adjusted, but the less absorptive power of the soil when 
brought in the dry form into contact with the solution is probably a 
much more potent cause. 

When 1,470 parts PO^ per million had been absorbed by the soil, 
the drained soil was treated with water in the manner previously 
described. The results are given in Table XVIII. 

Table XVIII. — Removal of absorbed phosphate from a clay soil by ivater. 







Total 






Total 






Total 


Volume 


Quantity 


quantity 


Volume 


Quantity 


quantity 


Volume 


Quantity 


quantity 


of perco- 


PO4 in so- 


PO4 re- 


of perco- 


PO4 in so- 


PO4 re- 


of perco- 


PO4 in so- 


PO4 re- 


late. 


lution. 


maining 


late. 


lution. 


maining 


late. 


lution. 


maining 






in soil. 






in soil. 






in soil. 


C. c. 


P. p. m. 


P. p. m. 


C. c. 


P. p. m. 


P. p. m. 


C.c. 


P. p. m. 


P. p. m. 






1,470 
1,420 


500 
60'' 


14 
9 


1,340 
1,330 


1,140 
1,330 


8 
11 


1,290 
1,270 


100 


47 


•220 


31 


1,380 


720 


7 


1, 320 


1,500 


9 


1,250 


320 


15 


1,360 


870 


7 


1,310 


1,650 


11 


1,240 


410 


15 


1, 350 


990 


9 


1,300 









26 



ABSORPTIOTSr OF PHOSPHATES AND POTASSIUM. 



The second column in the table indicates very strikingly the simi- 
larity in the removal of the absorbed phosphate from this soil and 
from the same soil in the preceding series, in that the solutions rapidly . 
run down to a practically constant concentration which is comparable 
with that obtained from the original soil by percolation with water. 
As in the preceding series, the amount of absorbed phosphate remain- 
ing in the soil when this constant concentration of the soil solution is 
reached is still large. 

In Table XIX are given the absorption results obtained with the 
clay loam. 

Table XIX. — Absorption of phosphate by a clay loam from a solution of sodium phos- 
phate, Na^HPOi, containing 200 parts per million PO^. 



Vol- 
ume of 
perco- 
late. 


Quan- 
tity PO4 
in solu- 
tion. 


Total quantity 

PO4 absorbed 

by soil. 


Vol- 
ume of 
perco- 
late. 


Quan- 
tity PO4 
in solu- 
tion. 


Total quantity 

PO4 absorbed 

by soil. 


Vol- 
ume of 
perco- 
late. 


Quan- 
tity PO4 
in solu- 
tion. 


Total quantity 

PO4 absorbed 

by soil. 


Ob- 
served. 


Calcu- 
lated. 


Ob- 
served. 


Calcu- 
lated. 


Ob- 
served. 


Calcu- 
lated. 


C.c. 
60 
100 
150 
260 


P. p. m. 

127 

19 

8 


P.p.m.. 

40 

120 

220 

430 


P. p. m. 

'"lib" 
220 

430 


C.c. 
360 
460 
550 
660 


P.p.m. 
5 
19 
32 

77 


P.p.m. 

620 

760 

910 
1,050 


P.p.m. 
640 
760 
920 

1,050 


C.c. 
770 
810 


P.p.m. 

71 
86 


P.p.m. 

1,180 

1,240 


P. p. m. 
1,170 
1,230 



These results are seen to be very similar to those obtained with the 
clay, the concentration of the solution rapidly decreasing to a very 
low value, and then graduall}^ rising. The passage of the solution was 
exceedingly slow in this case and a considerable length of tubing had 
to be added to the apparatus in order to secure the necessary pressure 
for forcing the solution through the soil. For this reason the experi- 
ment was stopped when only about 1,200 parts per million PO^ had 
been absorbed by the soil. The percolate was far from showing a 
saturated condition for the soil. An attempt was nevertheless made to 
study the removal of the absorbed phosphate by water, but when 
approximately 600 c. c. had passed the pressure necessary for filtration 
became so great that the experiment could not be continued in the 
apparatus used. The few results obtained are given in Table XX. 

Table XX. — Removal of absorbed phosphate from a clay loam by uuter. 



Volume 
of perco- 
late. 


Qnantitv 

PO4 in 
solution. 


Total 
quantity 

PO4 re- 
maining 

in soil. 


Volume 
of perco- 
late. 


Quantity 

P04in 

solution. 


Total 
quantity 

PO4 re- 
maining 

in soil. 


C. c. 


P. p. m. 


P.p.m. 
1,240 
1,170 
1,090 
1,010 


C.c. 
380 
490 
580 


P.p.m. 
54 
48 
46 


P.p.m. 
940 
910 
870 


90 
200 
300 


77 
75 
78 



DI80DIUM PHOSPHATE SOLUTION". 



27 



The results show the same general tendency as in the experiments 
already described. It will also be noticed that these results, together 
with those of the previous experiment, show conclusively that the 
washing-out process of the absorbed phosphate is essentiall}^ the same 
in kind, whether the soil has absorbed a large or a small amount of 
phosphate. 

The absorption results obtained with the fine sandy loam are given 
in Table XXI. 



Table XXI. — Absorption of phosphate by a fine sandy loam from a solution of sodium 
phosphate, Na^HPO^, containing 200 parts per million PO^. 



Vol- 
ume 
of per- 
colate. 


Quan- 
tity 

P04in 
solu- 
tion. 


Total quantity 

PO4 absorbed 

by soil. 


Vol- 
ume 
of per- 
colate. 


Quan- 
tity 
P04in 
solu- 
tion. 


Total quantity 

PO4 absorbed 

by soil. 


Vol- 
ume 
of per- 
colate. 


Quan- 
tity 

PO4 in 
solu- 
tion. 


Total quantity 
PO4 absorbed 
by soil. 


Ob- 
served. 


•Calcu- 
lated. 


Ob- 
served. 


Calcu- 
lated. 


Ob- 
served. 


Calcu- 
lated. 


C.c. 
50 
280 
330 
380 
470 


P. p.m. 
175 
130 
48 
62 
72 


P.p. in. 
10 
170 
250 
310 
440 


P. p.m. 

'""176"" 
.240 
310 
410 


C.c. 
580 
680 
780 
940 
1,090 


P. p.m. 
101 
105 
143 
152 
167 


P. p. m. 
540 
640 
690 

770 
810 


P. p.m. 
520 
600 
680 
770 
840 


C.c. 
1,320 
1,480 
1,700 
1,860 
1,960 
1 


P. p.m. 
165 
174 
174 
176 
195 


P. p.m. 
890 
940 
1,000 
1,030 
1,040 


P. p.m.. 
920 
960 
1,000 
1,020 
1,030 



These results are similar to those obtained with the other soils. The 
last results in the second column indicate that the soil is approaching 
a saturated condition, although only about 1,000 parts per million are 
absorbed. This is a much lower absorption than in the case of the 
monocalcium phosphate with the same soil. 

Table XXII gives the results for the removal of the absorbed 
phosphate from this soil. 

Table XXII. — Removal of absorbed phosphate from a fine sandy loam by ivater. 



Volume 
of perco- 
late. 


Quantity 

POiin 
solution. 


Total 
quantity 

PO4 re- 
maining 

in soil. 


Volume 
of perco- 
late. 


Quantity 

P04in 
solution. 


Total 
quantity 

PO4 re- 
maining 

in soil. 


Volume 
of perco- 
late. 


Quantity 

PO4 in 

solution. 


Total 
quantity 

PO4 re- 
maining 

in soil. 


C.c. 


P. p. m. 


P. p. m. 
1,040 
820 
650 
610 
580 


C. c. 

620 

750 

920 

1,010 

1, 110 


P. p. m. 
23 
20 
16 
14 
13 


P. p. m. 
560 
530 
510 
490 
480 


C. c. 
1,240 
1,340 
1,470 


P. p. m. 
13 
15 
15 


P. p. m. 
460 
450 
430 


130 
290 
400 
500 


166 

111 

32 

26 



Here again the concentration of the solution runs down rapidly, 
until when about 900 c. c. have passed the concentration becomes 
practically constant and comparable with that obtained when mono- 
calcium phosphate was used, although the absolute concentration is 
slightly higher. This shows in the case of this soil that the results 
with sodium phosphate differ from those with monocalcium phosphate, 
both as regards absorption and removal. 



28 



ABSORPTION OF PHOSPHATES AND POTASSIUM. 



The absorption and removal of phosphate for the fine sandy soil are 
given in Tables XXIII and XXIV, respectively. 

Table XXIII. — Absorption of phosphate by a fine sandy soil from a solution of sodium 
phosphate, Na^HPOi, containing 200 parts per million PO^. 



Vol- 
ume of 
perco- 
late. 


Quan- 
tity 
PO. 

in so- 
lution. 


Total quantity 

PO4 absorbed 

by soil. 


Vol- 
ume of 
perco- 
late. 


Quan- 
tity 
PO4 

in so- 
lution. 


Total quantity 

PO4 absorbed 

by soil. 


Vol- 
ume Of 
perco- 
late. 


Quan- 
tity 
PO4 

in so- 
lution. 


Total quantity 

PO4 absorbed 

by soil. 


Ob- 
served. 


Calcu- 
lated. 


Ob- 
served. 


Calcu- 
lated. 


Ob- 
served. 


Calcu- 
lated. 


C.c. 
200 
240 
3U0 
350 
400 


P. p.m. 
145 
22 
9 
33 
88 


P. p.m. 
110 
190 
300 
380 
440 


P. p.m. 

'"'igo' 

280 
350 
410 


C.c. 
460 
610 
690 
800 
970 


P. p.m. 
97 
90 
106 
159 
158 


P. p.m. 
490 
660 
730 
770 
850 


P. p.m. 
480 
630 
700 
780 
870 


C.c. 

1,120 

1,360 

1,550 

1,670 


P. p.m. 
145 
155 
190 
168 


P. p.m. 

930 

1,040 

1,050 

1,090 


P. p.m. 

940 
1,020 
1,070 
1,090 



Table XXIV". — Removal of absorbed phosphate from a fine sandy soil by water. 



Vol- 
ume of 
perco- 
late. 


Quan- 
tity 

P04"in 
solu- 
tion. 


Total 
quan- 
tity 
PO4 re- 
main- 
ing in 
soil. 


Vol- 
ume of 
perco- 
late. 


Quan- 
tity 

P04in 
solu- 
tion. 


Total 
quan- 
tity 
PO4 re- 
main- 
ing in 
soil. 


Vol- 
ume of 
perco- 
late. 


Quan- 
tity 
P04in 
solu- 
tion. 


Total 
quan- 
tity 
PO4 re- 
main- 
ing in 
soil. 


Vol- 
ume of 
perco- 
late. 


Quan- 
tity 

P04in 
solu- 
tion. 


Total 
quan- 
tity 
PO4 re- 
main- 
ing in 
soil. 


C. c. 


P. p.m. 


P. p.m. 

1,090 

960 

890 

830 


C.c. 
400 
520 
700 
900 


P. p.m.. 
25 
22 
12 
14 


P. p.m. 
800 
780 
760 
730 


C.c. 

1,040 

1,130 

1,220 

1,390 


P. p. m. 

10 

10 

9 

8 


P. p.m. 
720 
710 
700 
680 


C.c. 
1,490 
1,590 
1,670 


P. p.m. 

8 
7 
7 


P. p.m. 
680 
670 
660 


90 
170 
290 


145 
91 
52 



The second column of the absorption table indicates that this soil, 
like the preceding one, is approaching saturation at a much lower 
phosphate content than in the case with the monocalcium phosphate. 
The removal of the absorbed phosphate is the same as in the other 
soils, a practically constant concentration being reached when about 
1,200 c. c. have passed through the soil. This concentration is prac- 
tically the same as that in the case of the monocalcium phosphate. 

GRAPHICAL REPRESENTATION AND DISCUSSION OF THE RESULTS 
OBTAINED WITH SODIUM PHOSPHATE. 

In figure 4 the results of the preceding experiments on the absorp- 
tion and removal of phosphate are plotted for the soil. As before, 
the ordinates represent the quantity of phosphate absorbed by the soil, 
and the abscissas the volume of solution or of water which has been 
passed through the soil. The break in the curves gives the point 
where the phosphate solution was replaced by distilled water and the 
washing out of the absorbed phosphate begun. 

It is at once apparent that the sandy soil and sandy loam are 
approaching a condition of saturation, whereas the curves for the clay 
loam and clay soil are still rising rapidly and the soils are obviously 
far from saturation for phosphate. The greater absorption of the clay 



EESULT8 OBTAINED WITH DISODIUM PHOSPHATE. 29 

loam is, moreover, only apparent, being due entirely to the different 
starting points of the curves. When the starting points of the curves 
are made to coincide, the clay loam curve lies throughout below that of 
the clay soil curve, as it did in the case of the monocalcium phosphate. 
This will be more strikingly brought out in the application of the for- 
mula to these curves, when it will be shown that they are described 
by the same formula and have practically the same constants as those 
found to apply to the case of the monocalcium phosphate. There is, 
however, an actual inversion of the curves in the case of the sandy 
loam and fine sand, the former showing the less absorptive power in 
this case, whereas with the monocalcium phosphate it showed the 



1000 


CLAySO/L 

clay/loam 






^ 




S^NDYSOIL 




5 




.^,J-r— \ S/tND) 


'LO/IM 


■J 1000 




^^^-— -V^^''^ 




vl 




^ — K 








'^^ X 




^ 




^^ 


\ 


tt: 






— A~___^ 


Uj 






\7 " — * .^__^^ 


0, 






^\ """* — ' "-——.-_ 


[0 






^"^..__^^ 


^500 






^~^"~~~*~~~*— ~— .^^ 


5 









Fig. 4.— Soil curves. Absorption of phosphate by soils from a solution of disodium phosphate and the 
removal of the absorbed phosphate by vs^ater. 

higher absorptive power. Here again the difference between the two 
curves is much less than it appears to be, for if the starting points of 
the curves were made to coincide the curves would lie much closer 
together. The fact remains, however, that these two soils show a rela- 
tively different absorptive power in the case of the sodium phosphate 
from that shown where monocalcium phosphate was used. That the 
specific absorptive capacity of both these soils is markedly different in 
the present series than in the former series will be shown presently. 

The removal curves are in general the same as those in the monocal- 
cium series already given. With the almost saturated soils the curves 
drop rapidly at first, but soon descend at a slow and practically uniform 
rate. With the less saturated soils the drop is less marked, but the 
results, nevertheless, clearl}^ show that distilled water is slowly but 
constantly removing the absorbed phosphate, whether the soil is 
nearly saturated for phosphate or whether it is only partially satu- 
rated. This latter result was, of course, to be expected, since the 



30 



ABSOKPTIOJSr OF PHOSPHATES AND POTASSIUM. 



phosphate originally in the soils was found to be removed by distilled 
water at a practically constant rate. 

It has been found that the same differential equation describes the 
absorption curves in the case of the sodium phosphate as in the case of 
the monocalcium phosphate. It will be observed from the figure that 
the curves do not go to the origin, as they did in the case of the mono- 
calcium phosphate. This is due to the lower absorption at the very 
start of the experiment, either because of the lower absorptive power 
of the dry soil used in the present series as compared with the wet 
soil used in the previous series or because of the too rapid flow of the 
strong phosphate solution at the start before the apparatus was thor- 
oughl}^ regulated. Whatever may have been the cause, it is for this 
reason obviously necessary to modify slightly the constant log A in 
the equation 

log {A—y) — \og A—Kv 

by substituting log {A—y^^ where y^ is the ordinate of the point taken 
as the first reading, and also substituting the corresponding value of 
{v — Vq) for V, where % is the volume corresponding to the y^ taken as 
the first reading. The equation then becomes: 

log {A-y)^\og{A-y,)-K{v-v,). 

It is, of course, impossible to get any idea of the value of the spe- 
cific absorptive capacity of the clay soil and the clay loam from the 
two curves given in the figure, as these are far from showing any 
tendency to approach a horizontal asymptote. A careful examination 
of the figures, however, shows that when allowance is made for the 
different starting points of the curves the results fall almost exactly 
on the curve for the results obtained with these same soils by using 
monocalcium phosphate. In other words, the clay soil and the clay 
loam are quite accurately described by using the same value for A as 
in the case of the monocalcium phosphate, together with the other 
constants given below: 



Soil. 



Clay soil... 
Clay loam . 



5,500 
3,100 



log 



3.728 
3.474 



K. 



0. 000162 
. 000290 



The results for the sandy soil and the sandy loam are likewise quite 
accurately described by the above equation, using the following values: 



Soil. 



log 



Sandy loam 
Sandy soil . . 



1,100 
1,200 



2.968 
3.004 



0. C00680 
. 000669 



POTASSTUM FROM SOLUTIOT^ OF POTASSIUM CHLORIDE. 



31 



The results obtained by using these values are given in the fourth 
column of the tables for the respective soils. In the figure the absorp- 
tion curve is drawn through the calculated points, the experimental 
points being represented by the specific signs. The figure shows, 
therefore, what agreement exists between the results given in the 
third and fourth columns, respectively, of the absorption tables. 

In the following table is given a comparison of the specific absorptive 
capacities for phosphate found for the four soils when sodium phos- 
phate was used and when monocalcium phosphate Avas used, together 
with the constant K. The values of log A and log {A—y^) may be 
omitted in the comparison, as they depend on the above specific absorp- 
tive capacity and the starting point of the curves. 



Soil. 



Monocalcium 
phosphate. 



A. 



Sodium phosphate. 



Clay soil . . . 
Clay loam . . 
Sandy loam 
Sandy soil . 



5,500 
3,100 
2,800 
2,600 



0. 000158 
. 000267 
. 000299 
. 000247 



5,500 
3,100 
1,100 
1,200 



0. 000162 
. 000290 
. 000680 
. 000669 



It is at once apparent that the absorption of phosphate by the clay 
soil and cla}'^ loam studied is the same for the calcium and the sodium 
phosphate, as is shown by the equal values for the specific absorptive 
capacities for phosphate, A^ and the close agreement for the value of K 
in both cases. It is possible that the ferruginous nature of these par- 
ticular soils is responsible for this similarity in the absorption of the 
phosphate in the two cases. With the other two soils, however, the 
result is quite different. The use of the sodium phosphate has changed 
the specific absorptive capacity of both soils markedly, and also the 
value of K. It seems possible, therefore, that the sodium and calcium 
played an important part in the changes which have taken place in 
these soils. 



ABSORPTION OF POTASSIUM FROM A SOLUTION OF POTASSIUM CHLORIDE. 

For studying the absorption of potassium by soils a solution of 
potassium chloride containing 200 parts per million of potassium was 
used. Three of the soils studied were the same as those used in the 
phosphate experiments, namely, the clay soil, the clay loam, and the 
fine sandy soil. In addition to these, two other soils were studied — a 
loam and a sand. The loam was a sample of the Leonardtown loam, 
from Leonardtown, Md., and the sand a sample of Sandhill, from Dar- 
lington, S. C. 

The apparatus and general manipulations were the same as in the 
experiments already described. The soil was not previously washed 
with water, but was used in the air-dry form in which it had been 
kept for some weeks in the laboratory. The entire procedure is there- 



32 



ABSOEPTION OF PHOSPHATES AND POTASSIUM. 



fore comparable with that used for the sodium phosphate, where the 
dry soil was wet directly with the solution instead of previously by 
passing- water through it. The apparatus was then filled with the 
solution as already described and regulated so as to have a flow of 
about 50 c. c. in twent^^-four hours. The percolates were then ana- 
lyzed for potassium by the colorimetric method described in a former 
bulletin." In Table XXV are given the results obtained with the clay 
soil. 

Table XXV. — Absorption of potassium by a clay soil from a solution of jjotassium 
chloride containing 200 parts per million K. 







Total quantity 






Total quantity 






Total quantity 


Vol- 


Quan- 


K absorbed 


Vol- 


Quan- 


K absorbed 


Vol- 


Quan- 


K absorbed 


ume of 


tity K 
in solu- 


by soil. 


ume of 


tity K 
in solu- 


by soil. 


ume of 


tity K 
ineolu- 


by soil. 




















late. 


tion. 


Ob- 


Calcu- 


late. 


tion. 


Ob- 


Calcu- 


late. 


tion. 


Ob- 


Calcu- 






served. 


lated. 






served. 


lated. 






served. 


lated. 


C.c. 


P.%).m. 


P. p.m. 


P. p.m. 


C.c. 


P. p.m. 


P. p.m. 


P. p.m. 


C.c. 


P. p.m. 


P. p.m. 


P. p.m. 


50 


62 


70 


50 


410 


104 


520 


530 


750 


156 


760 


760 


160 


67 


230 


230 


500 


117 


600 


610 


890 


165 


810 


820 


210 


57 


300 


300 


550 


117 


640 


640 


970 


164 


860 


850 


260 


60 


370 


370 


610 


133 


690 


690 


1,140 


173 


890 


890 


320 


78 


440 


440 


690 


141 


740 


730 











In the first column appears the total number of cubic centimeters of 
solution passed through the soil, in the second column the concentra- 
tion in potassium of the successive fractions, and in the third column 
the total quantity of potassium absorbed by the soil. The fourth col- 
umn gives the calculated quantity of absorbed potassium obtained by 
a formula which will be described later. It will be noticed from the 
results in the second column that the first few hundred cubic centi- 
meters of the 200 parts per million solution in passing through the 
soil was reduced to a concentration of approximately 60 parts per mil- 
lion and that in succeeding fractions this concentration gradually rises, 
showing a tendency to reach the original concentration of the solution, 
until when about 1,100 c. c. have passed it is 173 parts per million. 
At this point the soil has absorbed nearly 900 parts per million of 
potassium. The absorption obtained with the clay loam is given in 
Table XXVI. 

Table XXVI. — Absorption of potassium by a clay loam from a solution of potassium 
chloride containing 200 parts per million K. 



Vol- 
ume of 
perco- 
late. 


Quan- 
tity K 
in solu- 
tion. 


Total quantity 

K absorbed 

by soil. 


Vol- 
ume of 
perco- 
late. 


Quan- 
tity K 
in solu- 
tion. 


Total quantity 

K absorbed 

by soil. 


Vol- 
ume of 
perco- 
late. 


Quan- 
tity K 
in solu- 
tion. 


Total quantity 

K absorbed 

by soil. 


Ob- 
served. 


Calcu- 
lated. 


Ob- 
served. 


Calcu- 
lated. 


Ob- 
served. 


Calcu- 
lated. 


C.c. 
50 
100 
350 


P. p.m. 
101 
99 
125 


P. p.m. 

50 

100 

290 


P. p.m. 

60 

100 

270 


C.c. 
500 

680 
880 


P. p.m. 
167 
164 
167 


P. p.m. 
340 
400 
470 


P. p.m. 
340 
410 
470 


C.c. 
1,060 
1,240 
1,390 


P. p.m. 

178 
175 
180 


P. p.m. 
510 
550 
570 


P. p. m. 
510 
540 
560 



«Bul. No. 31, Bureau of Soils, U. S. Dept. of Agr., 1906, p. 31. 



POTASSIUM FEOM SOLUTION OF POTASSIUM CHLOEIDE. 



33 



The absorption of potassium by this soil is not so great as in the 
case of the clay soil, nor is it so great as the absorption of phosphate 
bj^ this same soil; nevertheless the second column shows that the con- 
centration of the first fractions of the 200 parts per million solution 
is reduced to approximately one-half in passing through the soil. 
As more solution passes the concentration rises and slowly approaches 
the original value, having risen to 180 parts per million when about 
1,400 c. c. of the solution have passed through the soil, which at this 
point has absorbed nearly 600 parts per million of potassium. The 
results obtained with the loam are given in Table XXVII. 

Table XXVII. — Absorption of potassium by a loam from a solution of potassium chloride 
containing 200 parts per million K. 



Volume 
of perco- 
late. 


Quantity 
K in so- 
lution. 


Total 
quantity 
K ab- 
sorbed 
by soil. 


Volume 
of perco- 
late. 


Quantity 
K in so- 
lution. 


Total 
quantity 
K ab- 
sorbed 
by soil. 


Volume 
of perco- 
late. 


Quantity 
K in so- 
lution. 


Total 
quantity 
K ab- 
sorbed 
by soil. 


C.c. 
80 
110 
150 


P. p.m. 
100 
72 
69 


P. p.m. 

80 

120 

170 


C.c. 
210 
280 
320 


P. p.m. 
82 
90 
106 


P. p. m. 
240 
320 
350 


C.c. 
390 
460 
690 


P. p.m. 
120 
128 
150 


P. p.m. 
410 
460 
580 



With this soil the absorption is even more marked than with the 
clay loam, though less than with the clay. The first fraction is higher 
than the four succeeding ones. This is doubtless due to the same 
causes that produced the higher concentrations in the first fractions of 
the percolate obtained in the sodium phosphate experiments. This 
higher concentration of the first fraction is noticeable also with the 
two soils already described, although not so marked, and occurs like- 
wise in the case of the succeeding soils, as will be seen presently. 
After reducing the solution to approximately TO parts per million the 
concentration again rises until when about 700 c. c. had passed through 
the soil it was 150 parts per million. The experiment was not con- 
tinued beyond this point. In Table XXVIII are found the results 
for the sandy loam. 

Table XXVIII. — Absorption of potassium by a sandy loam from a solution of potassium 
chloride containing 200 parts per million K. 







Total 






Total 






Total 


Volume 


Quantity 


quantity 


Volume 


Quantity 


quantity 


Volume 


Quantity 


quantity 


of perco- 


K in so- 


K ab- 


of perco- 


K in so- 


K ab- 


of perco- 


K in so- 


K ab- 


late. 


lution. 


sorbed 
by soil. 


late. 


lution. 


sorbed 
by soil. 


late. 


lution. 


sorbed 
by soil. 


C.c. ■ 


P. p. m. 


P. p. m. 


C.c. 


P. p. TO. 


P. p. m. 


C.c. 


P. p. TO. 


P. p. m. 


50 


104 


50 


220 


141 


180 


740 


172 


290 


100 


96 


100 


300 


171 


200 


910 


180 


330 


150 


126 


140 


520 


185 


230 









The absorption in the case of the sandy loam is much less than in 
the soils so far described, but it is nevertheless quite marked in the 



34 



ABSORPTION OF PHOSPHATES AND POTASSIUM. 



first fractions of the solution passing through the soil. The absorp- 
tion by the fine sand, given in Table XXIX, is still less, but definitely 
traceable in its effects on the various portions of percolate. 

Table XXIX. — Absorption of potassium by a fine sandy soil from a solution of potas- 
sium chloride containing 200 parts per million K. 



Volume 
of perco- 
late. 



C.c. 
60 
110 
150 



Quantity 
K in so- 
lution. 



P. p.m. 
150 
141 

160 



Total 
quantity 
K ab- 
sorbed 
by soil. 



P. p. in. 
30 
60 
70 



Volume 
of perco- 
late. 



C.c. 
300 
540 
770 



Quantity 
K in so- 
lution. 



P. p.m. 
166 
168 

174 



Total 
quantity 

K ab- 
sorbed 
by soil. 



P. p.m. 
120 
200 
260 



Volume 
of perco- 
late. 



C.c. 

1,130 

1,390 



Quantity 
K in so- 
lution. 



P. p.m. 
183 
185 



Total 
quan tity 
K ab- 
sorbed 
by soil. 



P. p.m. 
320 
360 



REMOVAL, OF ABSORBED POTASSIUM BY WATER. 

The removal of the absorbed potassium from two soils was also 
studied, the soils used being the clay and the clay loam of the previous 
experiment. For this purpose the well-drained soils, containing 890 
and 570 parts per million of absorbed potassium, respectively, were 
washed by filling the apparatus with distilled water and continuing the 
percolation at the same slow and constant rate used in passing the 
potassium solution. The percolate, collected in fractions, was ana- 
lyzed for potassium as before. The results for the clay soil are given 
in Table XXX. 

Table XXX. — Removal of absorbed potassium from a clay soil by water. 



Vol- 
ume of 
perco- 
late. 


Quan- 
tity K 
in solu- 
tion. 


Total 
quan- 
tity K 

re- 
main- 
ing in 

soil. 


Vol- 
ume of 
perco- 
late. 


Quan- 
tity K 
in solu- 
tion. 


Total 
quan- 
tity K 

re- 
main- 
ing in 

soil. 


Vol- 
ume of 
perco- 
late. 


Quan- 
tity K 
in solu- 
tion. 


Total 
quan- 
tity K 

re- 
main- 
ing in 

soil. 


Vol- 
ume of 
perco- 
late. 


Quan- 
tity K 
in solu- 
tion. 


Total 
quan- 
tity K 

re- 
main- 
ing in 

soil. 


C.c. 


P. p.m. 


P. p.m. 
890 
820 
790 
770 
740 


C.c. 
220 
310 
450 
520 
720 


P. p.m. 
45 
29 
21 
19 
20 


P. p.m. 
720 
700 
670 
650 
610 


C.c. 
780 
840 
1,150 
1,260 
1,370 


P. p.m. 
19 
19 
22 
20 
19 


P. p.m. 
600 
590 
540 
520 
500 


C.c. 

1,510 

1,640 

1,760 

1,870 

2,110 


P.p.m. 
21 
19 
19 

17 
19 


P.p.m. 
470 
440 
420 
400 
360 


40 
80 
130 
170 


171 

78 
56 
47 



The rather high concentration of the small fraction collected at the 
start is due to the stronger solution contained in the soil. On passing 
more water the percolate rapidly becomes weaker in potassium, until 
when about 450 c. c. have passed the concentration shows a practically 
constant composition of about 20 parts per million, although the per- 
colation was continued until over 2,000 c. c. had been passed and the 
quantity of absorbed potassium in the soil reduced from nearly 9(»0 to 
about 350 parts per million. 

The removal of the absorbed potassium from the clay loam is shown 
in Table XXXI. 



RESULTS OBTAINED WITH POTASSIUM CHLOEIDE. 



35 



Table XXXI. — Removal of absorbed potassium from a clay loam by water. 



Vol- 
ume of 
perco- 
late. 


Quan- 
tity K 
in solu- 
tion. 


Total 
quan- 
tity K 

re- 
main- 
ing in 

soil. 


Vol- 
ume of 
perco- 
late. 


Quan- 
tity K 
in solu- 
tion. 


Total 
quan- 
tity K 

re- 
main- 
ing in 
soil. 


Vol- 
ume of 
perco- 
late. 


Quan- 
tity K 
in solu- 
tion. 


Total 
quan- 
tity K 

re- 
main- 
ing in 

soil. 


Vol- 
ume of 
perco- 
late. 


Quan- 
tity K 
in solu- 
tion. 


Total 
quan- 
tity K 

re- 
main- 
ing in 

soil. 


C.c. 


P. p.m. 


P. p.m. 
570 
510 
470 


C.c. 
200 
330 
470 


P. p.m. 
43 
31 
30 


P. p.m. 
440 
400 
360 


C.c. 
660 
720 

780 


P. p.m. 
30 

28 
30 


P. p.m. 
310 
290 
270 


C.c. 
830 
880 


P. p.m. 
26 
25 


P. p.m. 
260 
250 


70 
140 


89 
59 



The results in this case are very similar to those of the clay soil, 
although the washing with water was not continued so long. The 
concentration of the solution is higher, showing the more ready 
removal of the potassium from this soil. When 800 c. c. of water had 
passed through the soil the concentration was 26 parts per million and 
the quantity of absorbed potassium had been reduced from 570 to about 
250, whereas in the case of the claj^ the passage of a similar volume of 
water had reduced the soil from 890 to only about 600, and the con- 
centration of the percolate had already dropped down to the constant 
concentration of approximately 20 parts per million. 

GRAPHICAL REPRESENTATION AND DISCUSSION OF THE RESULTS OBTAINED 
WITH POTASSIUM CHLORIDE. 

In figures 5 and 6 are given the curves for the results obtained in 
the experiments on the absorption of potassium, as well as those 
obtained in the removal of the absorbed potassium by water. In fig- 
ure 5 are shown the results based upon the concentration of solution. 
The abscissas represent the volume of the solution or of water which 
has passed through the soil and the ordinates the concentration of 
potassium in the percolate. The upper boundary line of the figure 
gives the strength of the solution before passing through the soil, and 
the break in the curves gives the point where the solution was replaced 
by distilled water and the removal of the absorbed potassium was 
begun. Only the results for the clay and clay loam are given in this 
figure, as these are the two soils in whicli both the absorption and 
removal of potassium was studied. The absorption curve for the 
loam would lie between those of the clay and clay loam, the curves for 
the sandy loam and fine sand would lie above that of the clay loam. 
It is apparent from the figure that the general run of the absorption 
and removal of the potassium is very similar to the phosphate curves 
given in figure 2. The curves show that the concentration of the first 
few hundred cubic centimeters of the solution is materially reduced, 
in the one case approximately to one-half and in the other to one-fourth 
the original strength. With increase in volume of the solution passed 
the curves rise slowly and approach the upper boundary line of the 
figure (that is, the ordinate of the original strength of the solution) 
apparently as an asymptote. 



36 



ABSORPTION OF PHOSPHATES AND POTASSIUM. 



The removal of the absorbed potassium is very rapid at first, but the 
solutions soon reach a constant concentration, as is very strikingly 
brought out by the horizontal position of the removal curve in both 
cases. A comparison with figure 6 also shows that the concentration 
of the solution becomes constant at a point where only about one-third 
of the absorbed potassium has been removed. It is also noteworthy 
that the clay, although it contains throughout a much larger quantity 
of absorbed potassium, gives a lower concentration of potassium in 
solution than does the clay loam with its smaller potassium content. 
It follows, therefore, that the relative concentration of the potassium 

I 200 
^ 160 

^ 100 

S3 30 

Fig. 5.— Solution curves. Ab.sorption of potassium by soils from a solution of potassium chloride and 
the removal of the absorbed potassium by water. 





0.5 1.0 


1.5 


EO 




2.5 






3>0 


T^ 


^_ctflr^^^---Tr^ 


\^ 














- - 




" 











CLBY SOIL 




LITERS 



Fig. 6. — Soil curves. Absorption of potassium by soils from a solution of potassium chloride and the 
removal of the absorbed potassium by water. 



in the percolates gives no indication of the quantity of absorbed potas- 
sium present in the soil, although it must be admitted that this is in a 
form readih^^ soluble in water, as is shown by its continued removal 
by water in the above experiments. The magnitude of these absorp- 
tion results and the removal of the absorbed material show very con- 
clusively that in experiments dealing with the solubility of finely 
powdered substances of slight solubility, such as soils or rock-forming 
minerals, it is not so much the solubility itself with which one is deal- 
ing as with the decomposition and absorption phenomena and the 
removal of the absorbed products of decomposition. 

In figure 6 are shown the results expressed in terms of the soil itself, 
the abscissas being the volume of solution or water passed through 
the soil, and the ordinates the amount of potassium absorbed by the 



RESULTS OBTAINED WITH POTASSIUM CHLORIDE. 



37 



soil. It is at once apparent that the different soils show marked dif- 
ferences in their absorptive power for potassium. The clay loam, for 
instance, is approaching a saturated condition with a concentration of 
potassium in the soil at which the clay soil is still absorbing- at a rapid 
rate. Similar differences in the extent of the absorption are shown by 
the other soils, but the general run of the curves is the same as those 
of the more thoroughly studied clay soil and clay loam. The removal 
curves drop rapidly at first and then run downward in a straight line 
in the case of both of the soils studied. 

The similarity of the absorption curves to those of the phosphate 
given in figure 3, in that thej tend to approach a horizontal asymptote, 
is striking. It has been found that these results are quite accurately 
described by the same differential equation which describes the phos- 
phate-absorption curves. 



dv 



K{A-y) 



or integrating 

\og {A—y)=\og A— Kv 

where ^is a constant and A is the maximum amount of potassium 
the soil can absorb under the conditions of the experiment — i. e. , ^ is 
the specific absorptive capacity of the soil for potassium, y is the 
amount of potassium the soil has absorbed when the volume v of solu- 
tion has passed through the soil. In these experiments, however, as 
in the case of the sodium phosphate, on account of the fact that the 
curves do not pass through the origin, it is necessar}^ to substitute log 
{A—y^ for log JL, where y^ is the ordinate of the point taken as the 
first reading, and also to substitute the value of {v—v^ for v where v^ 
is the volume corresponding to the y^ taken as the first reading. The 
formula has been applied to the results obtained with the clay and the 
clay loam. When the following values are used a very good agree- 
ment between the experimental results and the calculated results is 
found to exist. 



Clay soil . . 
Clay loam , 



1,000 
650 



log 

(^-2/o). 



2.886 
2.740 



0. 000864 
. 000622 



The method of calculation is exactly the same as already described 
with the phosphate absorptions. The calculated results for the respec- 
tive soils are given in the fourth columns of the absorption tables. 
In figure 6 the absorption curves for the clay and the clay loam are 
drawn through the calculated points, the experimental points being 
indicated by the respective signs and the plots show, therefore, how 
well the calculated and found figures agree. 



38. ABSOEPTION OF PHOSPHATES AND POTASSIUM. 

Acknowledgments are due Messrs. H. C. Keith and R. M. Goss for 
assistance in carrying out analytical and other experimental details of 
the work. 

SUMMAET. 

The data presented in the foregoing pages throw much new light on 
the behavior of phosjDhates and potassium in the soil. The successive 
solutions obtained by slow percolation of water through the four soils 
have been shown to have a concentration in phosphate which is prac- 
tically a constant for any given soil, thus substantiating the experi- 
ments of Schloessing and other authorities, as well as the conclusions 
advanced in former publications from this Bureau deduced from 
entirely different lines of evidence. The soils studied, moreover, 
yielded solutions which differed little in concentration, except in the 
case of the Podunk fine sandy loam, which has a low absorptive 
capacity and is acted upon by water to an unusual extent. 

The absorption experiments with the phosphates have shown that at 
the start this is very rapid and complete, the strong phosphate solu- 
tion being reduced to the concentration characteristic of the water 
solution for that soil. This seems to show that the application of con- 
siderable quantities of a soluble phosphatic fertilizer would not mate- 
rially increase the concentration of the phosphate dissolved in the free 
soil moisture. On tracing the absorption of phosphate further, by 
the percolation of more phosphate solution, it has been found that 
while absorption continues it is becoming less marked and finall}^ a 
saturated condition will be reached. The law which appears to govern 
this change has been found to be that the quantity absorbed from a 
unit volume of the phosphate solution as it passes through the soil is 
proportional to the quantity which may yet be absorbed. This 
absorption process is mathematically represented by the differential 
equation 

where A" is a constant, A the maximum quantity the soil can absorb, 
and y the quantity it has absorbed when the volume v of phosphate 
solution has passed through the soil. The quantity A is defined as 
the specific absorptive capacity of the soil. The value of A differs 
considerably for the different soils, being highest in the clay and lowest 
in the sand}^ soil. In the case of the clay and the clay loam the same 
values were found with the sodium phosphate as with the monocalcium 
phosphate, but in the fine sandy loam and fine sand these two phos- 
phates gave entirely different results, the specific absorptive capacity 
with the monocalcium phosphate being over twice as great as with the 
sodium phosphate. 



SUMMARY. 39 

The removal of the absorbed phosphate appears to be in general the 
same as the removal of the phosphate from the original soil. The 
concentration of the percolates becomes constant when onl}'^ a 
fractional part of the absorbed phosphate has been removed, and this 
concentration is practically that yielded by the original soil with a 
far less phosphate content. It would therefore seem that the con- 
centration of the phosphate in the soil solution is practically the same, 
whether the soil contains a large or a small quantity of absorbed phos- 
phate, and that it is this absorptive power of the soil which controls 
the concentration of the phosphate in the free soil moisture. It fol- 
lows that with change in the absorptive power of the soil the concen- 
tration of the phosphate in the free soil moisture would also change. 
Attention has been called to the unusual extent to which the Podunk 
fine sandy loam is acted upon b}^ water. It is therefore interesting 
to note that the concentrations of the phosphate in the aqueous perco- 
lates from this and the other three soils came closer together after the 
soils had been treated for some time with the calcium or sodium phos- 
phate solution than they did before this treatment. This seems to 
indicate that by the similar treatment of the soils with considerable 
quantities of the same chemical substances the soils have been made 
more alike in their chemical behavior toward water, so far as phos- 
phate — the only constituent studied in this case — is concerned. It is 
also noteworthy that the absorbed phosphate is not insoluble, but is 
slowly and continuously diffusing into the free soil moisture and 
becoming, therefore, directly available to plants. 

The results obtained in the potassium experiments, while not so 
comprehensive, nevertheless show the same general tendencies. The 
five soils studied for their absorption all show a more or less marked 
absorptive power for potassium, which is, however, considerabl}^ less 
than the absorptive power of these soils for phosphate. The law gov- 
erning the absorption process appears to be the same with potassium 
as with phosphate, and the curve of the absorption is quite accu- 
rately described by the same differential equation: 

, The constancy of the removal of the absorbed potassium by water 
is even more striking than in the case of the phosphate and the con- 
clusion that the concentration in the free soil moisture is dependent 
on the absorptive power of the soil is well sustained by these results. 
The absorbed potassium, like the absorbed phosphate, is continually 
diffusing into the free soil moisture and becoming, therefore, directly 
accessible to plants. hr 

o 



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